One common way sci-fi writers try to make their aliens seem more 'alien' is to give them an extra heart.
This has been done for example in Babylon 5, Dr. Who and Alien Nation among others.
But: Would a humanoid (note: in this question humanoid only means two legs, two arms, one head, all in ascending order) really have any obvious benefits from this biology? Is there a way two hearts could actually be detrimental?
It might be better not to think in terms of hearts but chambers. Especially since to a fish, we might be said to have two hearts. Basically, with the evolution of lungs, we evolved one 'heart' for the lungs and one 'heart' for the rest of the body, to keep oxygenated and deoxygenated blood in separate systems (unlike amphibians which have greater mixing of the two types of blood). So if the need to keep the two types of blood (oxygenated and deoxygenated) lead to the evolution of the 4 chambered, dual fish heart system, perhaps there are four types of blood needed to be kept separately leading to the 8 chambered, dual human heart system.
Edit: Some info/citation
Edit2: 
Since heart disease is the leading cause of death in the west, having two hearts has seemingly obvious advantages.
However, heart disease isn't necessarily a result of wear and tear on the heart (though it certainly can be, especially if pumping against great pressure, e.g. high blood pressure or pulmonary hypertension.)
Commonly heart disease that kills people is coronary heart disease, that is, a buildup of plaque in the arteries supplying oxygen to the heart muscle. In humans, a redundant heart might help if the first one suffers, but in all likelihood, plaque would build up in the arteries of both hearts, placing both of them at equal risk of being diseased and giving out more or less at the same relative time.
In humans, therefore, there would be little seeming advantage to a redundant heart. A potential disadvantage is the nutrients and oxygen requirements of the being would be higher for no particular gain.
However, if parts of the body advantageous to survival (e.g. muscles) depend on high flow of oxygenated blood, such an individual may be able to better endure physically demanding tasks. That scenario would also depend on the lungs not being the limiting step in blood oxygenation.
In humanoids, if you double the lungs (four lungs) and double the heart, this might give them an advantage. It would also require a much larger chest, though, or devoting another part of the body to additional lungs, which would make the body more fragile and less flexible. The chest is already fairly fragile, requiring a protective cage of ribs, and an injury here is more dangerous than in some other areas of the body. Having more ribs would limit flexibility (your chest isn't very flexible), so that doesn't seen to grant any survival advantage.
Overall, then, I don't think a humanoid form would be better served by two hearts.
In evolutionary terms, the only creatures that needed auxiliary hearts have been giant sauropod dinosaurs, who had to pump blood up considerable distances to the heads. Even then, there wasn't a real second heart, but rather modified structures to ensure blood pressure could be maintained to the head, and also to ensure there would be no damage should the animal lower the head suddenly. Giraffes have analogous structures.
If the creature is exceptionally large, long or tall, then there would be some sort of need for a true second or multiple hearts, but we are talking about creatures the size of Dune Sandworms, or some sort of supersauropod far taller than any discovered to date. It seems that for any creature on Earth using conventional biology, one heart is sufficient.
A jawed leech has two long, segmented hearts, one running up each side of the body. An earthworm has five ring-shaped hearts, which sit in the seventh through eleventh of its hundred and fifty sgements. Although neither of these creatures is closely related to any humanoid animal, I can certainly imagine a humanoid on some alien tree of life ending up with a similar circulatory plan.
As Thucydides mentioned, some people speculate that giant sauropods may have had auxiliary hearts, although there's no direct evidence for this. Many animal lineages on Earth, including theropods** and sauropods, have undergone striking changes in shape and size. Vestigial structures are common, and the distinction between arms and forelegs is blurry. Thus, on a world where auxiliary hearts actually existed, I could easily imagine a humanoid creature that has an auxiliary heart because it descended from much larger quadrupedal ancestors.
Level River St mentioned that an octopus has two branchial hearts, which move blood through its gills, in addition to its main "systemic" heart. Other coleoids, including squid, cuttlefish, and vampire squid, have the same extra hearts, whose advantages seem to be well understood. Coleoids vary a lot in the numbers and roles of their arms. In some lineages, an arm pair has gone missing, or turned into a pair of sensory filaments not used for grasping. In other lineages, one pair of arms has turned into a pair of tentacles, which are often much longer than the other arms. In the distant future, I suppose some coleoids might end up with one pair of arms, one pair of tentacles, and zero to six pairs of sensory filaments or other non-grasping appendages. If that happens, we could have three-hearted humanoids right here on Earth.
** Under your definition, by the way, I would classify theropods as clearly humanoid!
Aside from redundancy, if a single heart could not be capable of regulating high blood volumes or weights, then this would necessitate the presence of a second heart. The hearts might be contained in dichotomised, greater and lesser, circulation, work in tandem in a single circulatory system, or work in a combination of the two.
Separate hearts can also be considered for the addition of a secondary or equivalent fictitious type of blood, or a liquid analogous to blood that might be inherent to the circulations of certain figures, or as the result of an environment.
(From a certain point of view, we already have two hearts. But let's ignore that sophistry.)
Naively thinking, two (or even three or four) hearts might indeed have a significant advantage in terms of blood pressure.
There would almost certainly be no advantages in terms of redundancy.
Coronary heart disease as well as mechanical wear on valves would arguably affect both hearts in the same way, so there is little to gain. A trauma destroying one heart would likely either cause death by exsanguination (puncturing trauma) or would have a good chance of destroying the second heart as well (blunt trauma).
In any case, the backup heart would not be very well-adapted to suddenly having to handle twice the load, either.
However, the pressure advantage remains, if you ignore the practical "implementation problems":
As it stands, the heart generates a short, extreme peak pressure during what we call systole (around 130mmHg on average, but quite possibly twice as much in some conditions) followed by a period of no pressure at all.
Ironically, the heart itself needs blood to survive, too, and blood will only flow through the coronary arteries during the diastole. So, someone, somehow, has to make sure this works. Blood generally has to keep flowing in a kind of steady way.
To add to this, certain organs -- kidneys most notably, but also the brain -- require a certain minimum pressure and a certain average pressure (of around 70mmHg if I recall correctly) or they will cease to function. This is one of the causes that may e.g. result in the phenomenon known as "shock kidney".
These requirements are a considerable challenge to the arteries which are responsible for keeping that beast running. They must be both muscular and elastic, they must be able to adapt to changing conditions rather quickly and invisibly (you have no idea how much work is necessary when you stand up, just so your brain won't suddenly stop working!), and they are subject to a lot of wear and tear and calcification.
Wouldn't it be nice if we could make the lives of our arteries a bit easier? It turns out we can.
Given two hearts which beat anti-synchronously (or even three hearts running in three-phase mode), in order to achieve the same average pressure, the peak pressure could be much lower. The pressure curve would naturally be much smoother, on a more even level. The problem of having to maintain a pressure minimum would magically go away.
This is comparable to an "ordinary" four-stroke car engine in comparison to a six-cylinder or an eight-cylinder, or to a "typical" motorbike engine with one or two cylinders on the other hand side. The "more cylinders" engine will always run a lot smoother, with less noise and vibrations (when assuming the same power, obviously a 10-15x more powerful motor will make more noise!), and with less overall tear and wear.
Now, the problem is getting two hearts to beat in sync, anti-synchronously. If they get out of sync, you will see more or less pronounced interference effects (as known e.g. in electrics or acustics). In the average case, you would have a sub-optimal pressure / blood flow. In the worst case, you would have the pressure waves add up, resulting in a pressure spike twice as high as anticipated. Unluckily, due to the nature of asynchronous waves interfering, that worst case is not just possible, but guaranteed to happen regularly.
Thus, you would have to build a system that guarantees that the hearts are, and stay, in sync. Rather than having their own SA node each, there would have to be a single SA node for both hearts, and a "delay circle" of sorts for one heart which delays the signal by exactly one half period length. Of course, the delay circle would have to be communicated from the SA node since it needs to adapt its delay with the heart rate.
That's a very complex system with a lot of logic, which is prone to failure. The two hearts would arguably need to be physically separate (rather than just doubling the number of chambers) in order to retain the "wear and tear" advantages. That would however place the SA node (which is a single point of failure) somewhere in between them, presumably in fatty tissue, making it again rather vulnerable to external injury.
If anything happens to this complex system, it all blows up.
The hearts that we currently use have as-dumb-as-can-be designs, which is what makes the design so ingenious. There are no "super clever" bits in them with a lot of synchronization and steering logic. It's membrane potentials spontaneously building up, and firing, and a few conducts and insulators that direct the wavefront in the desired shape. Yes, there are a few regulation mechanisms which can slightly moderate (but not substantially change) the function of certain parts in the conductive system, but all in all it's a hardwired, stubborn little thing that does exactly what it does during your entire life.
There is hardly anything that could go wrong, and this is a good thing. A heart rarely ever fails. You could say it happens on average once in a life.
One alternate reason for a sci-fi species to have an additional heart would go along with another fairly common sci-fi trope; Heavyworlder.
This is also related to preserving proper blood pressure to the brain. Like several other answers, a valve system could also do this, but a second heart could help normalize the blood flow against a high local gravity.
For a dual heart system to exist it would need to provide a benefit like you say, some may include:
Survive a natural predator whose deathly attack in the heart to all the proto-hominids. Let's not forget that homo sapiens is for one reason or another one of the only living species of the genus homo. So on another planet the leading humanoid species may have many other species maybe not as intellectually capable within the same genus. Having developed the second heart could be the reason this species became the dominant one on that planet.
Another one as pointed by Plinth, could be to provide alternatives paths for blood. In our body the blood vessels expand to increase the blood influx the parts of the body which most need it (or contract for the opposite effect). A second heart could provide the benefit or a much faster reaction than vasodilation and/or a broader range (total switch on/off vs tuning). If the planet/original environment was susceptible to very abrupt temperature changes which could cause severe hypothermia to biology like ours and where being able to shut off completely within seconds blood supply to skin, internal organs (digestion,...), and keep brain and running muscles oxygenated twice as fast as normal could provide a background for a second heart which would stop pumping at once and let the second heart pump twice as fast a it could if the other heart was also pumping. Other benefit could be to switch the skin breathing on/off (like bactractians) in presence of toxic gases, or when plunging in water, ...
That second heart being present would then have slowly evolved to become fully redundant with the first one as the humanoids progressed in evolution as a nice additional extra benefit.
You also ask the possible downfall of having a second heart. Like in many systems, the big problem of redundancy is cost:
those are of course only a few ideas /suggestions
As Aarthew III pointed out, the body would relying on both hearts until one failed, this would result in the second heart straining to become stronger if the first failed. This would result in (most likely) intense physical training after the first failed. Two solutions come to mind (that haven't been said already);
We could split the work of filtering blood in two hearts if need be, as we do with lungs and kidneys. In that case, it could even happen that we may donate one of the hearts if we could survive on the other(we can donate one kidney if they are in good health, and one is enough to sustain us). But since this is a hypothetical scenario, it would be speculation.
It is said that dinosaurs used to have eight hearts. This could be due to their enormous size and long neck, so additional hearts would be needed to pump blood all the way to the head. Maybe after a few hundred thousand years, if humans needed long bodies or something like that, we may evolve to have extra hearts.
If humans can be argued to have two hearts (because four lobes), then an example of, essentially, six hearts in a human-sized mammal is the horse.
Now, this is NOT a humanoid, but the same requirements that require it to have frogs in its feet could drive development of lower-limb hearts in bipedal creatures. In fact, it's even a little surprising that, once we became bipedal, we did NOT develop something like this - instead we had to develop considerably improved blood supply structures to the brain to prevent blacking out when we stand up, and it's still a problem for many of us. One of the many small things that somewhat supports the aquatic ape hypothesis, I feel.
So in short; yes, additional hearts or compressible chambers around the body, either mechanically operated (in sole of foot or back of knee) to automatically cause movement to push blood around the body, seems like it would definitely be an advantage in any large-brained biped which had to switch between long periods of lying prone and periods of rapid dashing.
Also, brains. Blacking out is inconvenient. A separate chamber or entire heart devoted purely to feeding blood to the brain seems like it would be useful anywhere that blood pressure to the head will often change significantly, whether by changing attitude (stand/lie), acceleration, or injury.
So you could argue that Klingon-type warrior races would likely have brains protected by a very thick skull and a largely isolated blood supply, the better to survive in combat for longer; as might flying bipeds that pull high g-forces.
One of my fantasy creatures has two hearts. The extra one is only active during fight or flight situations so extra oxygen is sent to the brain. This creature is also highly regenerative so the backup heart could be used while the other is repaired.
Here is what I came up with
*The only way it could possibly be helpful is if you made them alternate. This would make them like kidneys as if you lost one the other could take over. This would however cause some heart strain I'd you lost one as the remaining heart would have to take time to build up the strength to do twice the original work. However there is also more potential for your alien athletes. It also explains how Dr.Who is able to run so well!
*Both working at the same time at the same strength would be very dangerous. Just look up "why giraffes heads don't explode".
Edit: Thanks to Michael I also came up with this. You could have both work at the same time but they would be very weak. If they were too strong then the giraffe head happens. If they were weak there would be no point in having two hearts. As if you lose on e you die. So get them just weak enough to give you minor problems if you lose one but not so strong that your head explodes.
