Is this interpretation of "stacking" temp HP RAW?

Question

Assume you cast False Life at 1st-level, gaining 5 THP, we'll call this one False Life A. You then cast False Life at 1st-level again, gaining 8 THP, we'll call this one False Life B.

Bolstering yourself with a necromantic facsimile of life, you gain 1d4 + 4 temporary hit points for [1 hour].

The rules on Temporary Hit Points state that:

Healing can't restore temporary hit points, and they can't be added together. If you have temporary hit points and receive more of them, you decide whether to keep the ones you have or to gain the new ones. For example, if a spell grants you 12 temporary hit points when you already have 10, you can have 12 or 10, not 22. Unless a feature that grants you temporary hit points has a duration, they last until they're depleted or you finish a long rest.

So, in this scenario, you decide whether to gain 5 THP from False Life A or 8 THP from False Life B. Let's assume you decide to gain the 8 THP from False Life B. The section on Temporary Hit Points does not specify that False Life A ends. The THP from False Life A most likely weren't "depleted," as the rule never specifies that they would be, and False Life has a duration of 1 hour, which means it ignores the section about the effect ending once depleted.

The rules on Combining Magical Effects state that:

The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect--such as the highest bonus--from those castings applies while their durations overlap, or the most recent effect applies if the castings are equally potent and their durations overlap.

Since False Life B grants 8 THP, compared to False Life A's 5, False Life B would be the most "potent" effect, and therefore "applies" its effects, granting you 8 THP. Assume you then take 4 damage, and the 8 THP granted by False Life B becomes 4 THP, False Life A would then become the most "potent" effect, applying its effect, and granting you 5 THP, as False Life B's 4 THP no longer applies, but enters the state with which False Life A was in previously, waiting to become the most "potent" effect once the THP granted by False Life A drops below 4, effectively "stacking" temporary hit points.

Using a strict reading of the rules, is this RAW?

Answer 1

No

You are running afoul of the Specific Beats General rule, and also are trying to torture the plain English of the specific rule in order to get the result you want.

So first, the rules on combining magical effects are general rules. The rules on combining temporary hit points are specific. So forget about the general combining rules, they are inapplicable, overridden by the specific rules for temporary hit points.

And those specific rules say, in so many words, they don't combine. You get to have the points from one spell or the other-- your choice-- but not both. The wording doesn't say in words that (in your scenario) that the five old temporary hit points disappear, but it is unambiguously implied in the structure of the rule:

you decide whether to keep the ones you have or to gain the new ones.

Emphasis mine: That "or" in a plain language reading is an exclusive or. One branch of the or, or the other. Not both. If, as in your example, you do gain the new ones, you do not keep the old ones. Therefore, they are gone. They are not held in some reserve to benefit you under any condition.

Answer 2

It won't work to stack them

My first instinct was to say "no", because the rules intent is entirely clear: no messing around with tricks to stack multiple instances of temporary hit points together. When you gain new temporary hit points, they replace the ones you have. However, there is a finer point here.

There is no conflict between a general and a specific rule. Both rules are general rules: how to handle the exact same effect, and how to handle it when you can choose between two sets of temporary hit points. However, in this special situation you never get to choose which ones to keep:

false life is not an instantaneous effect; it has a duration of 1 hour and says:

Bolstering yourself with a necromantic facsimile of life, you gain 1d4 + 4 temporary hit points for the duration.

That is, the temporary hit points it provides are not "yours", granted by an instant effect once and then sticking to you. They are sustained by the false life spell for the duration.

Here, both sources of temp hp have the same name, and thus the moment you cast a new version of the spell, the weaker version of the two gets suppressed. You do not have any temporary hit points from the weaker version to decide on then. For the duration of the stronger version, you have only those temporary hit points. Once the stronger version goes away, the weaker version kicks back in, and for its remaining duration, you benefit from its temporary hit points.

That means, if you first cast a false life at level 2 for 1d4+9 hp, and one minute later one of level 1 for 1d4+4, and get 10 hp from the first and 8 hp from the second casting, the second casting will be suppressed until the first one runs out. Then, for one more minute, you can enjoy 8 temporary hp.

Note however, that the first spell does not end when your temporary hp from it are removed by damage. That is, it still is the stronger effect, and will run its full hour. You only will get the 8 hp from the second casting for the last minute, unless someone casts dispel magic on the effect of the stronger casting and removes it. That makes this tactic essentially a waste of spell slots.

That is because the higher-level casting is the more powerful effect, as per PHB, p. 201:

A spell’s level is a general indicator of how powerful it is

and when you upcast false life, it counts as a higher-level spell (same page):

When a spellcaster casts a spell using a slot that is of a higher level than the spell, the spell assumes the higher level for that casting

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P.S. As an aside, even if this worked, it would be a weak use of your spell slots, mostly because false life is a pretty weak spell to begin with.

In effect, the main difference between this and just upcasting false life would be at best 3 extra temporary hp per slot, if you roll lucky. But even at that rate, this spell is not great. It typically gives you fewer hit points per spell level than a cure wounds cast by a competent caster, and you have to spend it up front, not save it for when you actually take damage.

I have it for my wizard, and would much rather conserve the slot for shield, which can prevent a lot more damage when it cancels a strong hit, and isn't wasted when there is no fight within the next hour.

I'd maybe consider memorizing false life to cast it in the morning before I get my slots back for maybe one hour of extra safety instead of those slots just going to waste, but to do that I would have to have it prepared and normally this use is not worth a preparation slot. The other use of false life I'd consider is putting it into a contingency, and for that you would want the strongest, most upcast version you could afford.

Answer 3

The opposite of 'keep' is 'lose'

You have a binary choice

You have one casting of false life running and you cast another one. Now you have two different sources of temporary hp. The rules on this are clear:

temporary hit points...can't be added together. If you have temporary hit points and receive more of them, you decide whether to keep the ones you have or to gain the new ones. For example, if a spell grants you 12 temporary hit points when you already have 10, you can have 12 or 10, not 22.

You have a choice: to keep the ones you have or to gain the new ones. The "or" indicates a binary choice; you must do one or the other. And making one choice explicitly rejects the other option. If you do not gain the new hp, you lose them - lose in this context is the opposite of gain. If you do not keep the old hp, you lose them - lose in this context is the opposite of keep. The rules do not say 'you decide whether to use the ones you have or to use the new ones'. They do not say that the hp you rejected when you made your choice were maintained or set aside. Your choices are keep (and if not, lose) or gain (and if not, lose).

You are running two spells

You are correct that you are now under the influence of two spells with the same same, and the rules on simultaneous effects also apply. The duration of the false life spell is 1 hour, and nothing in the spell description states that it ends early when the temporary hp are lost, whether you are losing them by taking damage or losing them by gaining temporary hp from another source. However, just because the first spell is still running does not mean you can retain the hp from it when you have used some or all of the hp from the second spell. In this sense it is much like having only one spell running and then taking damage. Even if all the the temporary hp the single spell provided have been used, the spell itself persists for the remainder of the hour. Spells last until their end conditions are met regardless of whether or not they are producing effects, and the only end condition of false life is the duration.

"Most potent" is determined at the time of casting, only

You posit that you can retain hp from one casting of the spell, and set these aside in reserve when you cast the spell again for a greater amount. Then, when you have depleted the hp from the second spell, the first spell would become the more potent one and resume its effect. Even if you could 'reserve' hp from the first spell (and you can't), this would still be incorrect, because the two spells are not continually checking to see which is the more potent. Rather, their potency is is compared at the time of casting, and which spell is active and which suppressed is decided at that point, and does not subsequently change. The rules about "most potent" are not at all clear, but this is one thing the DMG errata does clarify:

...when two or more game features have the same name, only the effects of one of them—the most potent one—apply while the durations of the effects overlap. For example, if a target is ignited by a fire elemental’s Fire Form trait, the ongoing fire damage doesn’t increase if the burning target is subjected to that trait again.

Fire form does a d10 of damage each round the creature is on fire. If I am subjected to two of these, I don't roll both and take the higher one. The 'potency' of the fire is '1d10' and all applications of it have the same potency. Similarly, the 'potency' of false life is '1d4+4'. This is true for any first level casting, regardless of what was subsequently rolled, regardless of whether the temporary hp were later lost to damage, and regardless of whether the hp were 'kept' or lost when the second spell was cast. If the second spell was cast at second level, that would be a more potent effect (even if the total hp rolled was less).

So, assuming you could 'set aside' the hp from the first spell when you cast the second (which you can't - they are lost), when the second set of hp had been removed by damage, the hp from the first cast would not re-assert themselves, because the second spell was still in effect. If that second cast was then dispelled, then you would indeed be under the effect of the first spell, and could use those hp (again, assuming you had been able to retain them, which you couldn't).