How to handle levels when adding PCs between Lost Mine of Phandelver and Storm King's Thunder?

Question

(If it's not obvious from the title, answers to this may have Storm King's Thunder spoilers, and possibly a couple Lost Mine of Phandelver spoilers.)

Some background

My regular playgroup consists of me as the DM with two players (my wife and oldest child), and for a variety of reasons this group isn't likely to get more players in the near future. We're about to wrap up the Lost Mine of Phandelver, and (as they haven't taken many of the optional quests) they'll probably end up partway through 4th level. It's been a bit of a challenge at times, as the adventure is designed for four characters and we only have two. I've been having an NPC with them at times to help, and have reduced the size of encounters at times, and we're making it work well enough, but it's clear that it'd work better with a full party.

We're planning on playing Storm King's Thunder next. And as it also is designed for four characters, we're thinking that we want to try out each player controlling two characters. (In practice my wife has been controlling two characters a lot of the time anyway, as she's been directing the actions of NPCs going along with the party.) So we have these existing characters that the players are using, and want to introduce a new one for each of them to also control.

What level do I start these new characters at?

The main options I see are to either

1. Create these new characters at roughly the same level as the existing ones, add them to the party, and follow the standard transition listed in Storm King's Thunder to start them in chapter 2 of the adventure.
2. Create these new characters at level 1, and find a way to get the party from the end of Lost Mine of Phandelver to the start of chapter 1 of Storm King's Thunder.

Especially as we're all still pretty new to the game and the various classes, we kind of want to start the new characters at level 1 so that the players have time to get used to the abilities of the new characters slowly as they adventure, just like they did for the characters they currently have.

I think this is different from "Should I boost a new player's character level to match the rest of the group?" because in this case the new characters will be played by the same players as the existing characters. Also, the existing characters are both spellcasters (1 a Wizard, 1 a Sorcerer), and we would probably be adding something that gets larger hit dice to help round out the party, which may mean that it won't take many levels for the party to have a similar number of maximum hit points.

Questions

While I think I know how to handle if we go with option 1 (as it's more-or-less the expected flow from one adventure to the other), my questions are about how to handle if we go with option 2.

1. Will it "work" to do the first chapter of Storm King's Thunder with two of the four characters starting at level 4 or 5 rather than 1? That is, will the new characters have a chance to "shine", or will the existing characters just completely overpower them such that they might as well not have come along expect to pick up the XP from the encounters? Or will I encounter other problems or something not being fun?
2. I was thinking of doing a more traditional XP-for-encounters based advancement rather than the milestone-based leveling listed in the Storm King's Thunder book. Will there be ongoing problems beyond the first chapter with some characters in the party having started at and probably continuing to be a higher level?

Or is this all just a terrible idea and I should go with Option 1, or with all new characters entirely, or with some option I haven't considered?

Answer 1

Either will work fine.

First, I hope it's obvious that creating L4 second PCs for each player and starting during chapter 2 poses neither narrative nor mechanical problems--it's basically exactly what's intended.

As for starting at chapter 1 with new first-level characters, Storm King's Thunder is designed to start with a full party at level 1. At worst case you'll have two L5 and two L1 PCs; the 5s will grossly outpower the 1s, allowing you to breeze through encounters pretty quickly. But Those L5s will be gaining little 25XP and 50XP increments, which won't do much for them; the L1s will be L3 within a couple of days and, for the rest of the campaign, will alternate between lagging by one level or being at the same level. (The XP progression has you in mind! Your second set of PCs may even be 4th level before the first set hits level 5--look closely at the XP chart on PHB15.)

That said, I advise starting the two new PCs at L1.

If I recall correctly, your two players are relatively new to the game. Managing two PCs isn't a crazy task, but using the first few levels as designed--to teach them the abilities of that PC--is going to be a useful construct. (See "Tiers of Play," PHB 15)

Answer 2

Start them at level 1

While it seems clear that the point is to help supplement the issues present with the current adventuring duo, you also mention that these are fairly new players. As such, having the time to learn how to control the new characters would likely prove very beneficial to them.

That being said...

My suggestion would be to NOT immediately start into Storm King's Thunder, but rather to treat the new characters as if they were actually new players whose intention is to join an existing party without being underpowered. I think that the best way to solve that, without just leveling them immediately, would be to run the two new characters through a short adventure or two and allow them to level in a more natural fashion.

This will be a good way to give your players time to not only learn the abilities of the new classes, but also to give them time to get into the mind's of the characters themselves before they are thrown into having to control both of them at once. There are numerous one-shot style adventures that can be downloaded for free which will serve this purpose. Just pick something simple that gives the new characters a chance to use their specific skills.

I recommend this from my anecdotal experience of preparing to do this exact thing for my wife and oldest child. My wife has played a few times in solo sessions now and has some experience with handling two characters at once, but still prefers time to understand the different classes before jumping into, and committing to, them. Our oldest has played some simple RPGs with us before and has been learning D&D, but needs time to grasp everything before really getting into a full adventure.

Answer 3

Don't start them at level 1

It seems you want to add some meat shields to the squishies. Starting them on first level, with fewer hit points than the casters kind of defeats the purpose. Keeping them alive would be an extra burden on the casters for the first few levels.

Assuming a quite reasonable Con value of 12 for a Wizard, he has 22 hit points on level 4, equal to a second level Fighter with 16 Con.

What to do

We have tried using two characters, you do not get as attached to both as you would to a single one. Considering they do not even start at the same time, I assume the second set will be considerable less important for the players. They might even be treated as NPCs (run by players).

If you want to emphasise this difference, start them one or two levels below the existing characters.
Otherwise start at the same level.