For those wondering, the Bloody Nine is a brilliant character in the "first law" trilogy by Joe Abercrombie, who is basically a fighter called Logen Ninefingers until he starts taking hits, and then his alter-ego the bloody nine manifests as an almost-demonic possession who will kill basically everyone in the place, damage fuels his wrath, and has the strength of mountains, etc. he's pretty awesome. He burns out fast, leaving logen with all the wounds he picked up, usually in a lot of pain or passing out. He's an awesome character. Another example would be "flippy" from happy tree friends.

So, I am pondering the mechanics for involuntary character switching for dual-personality characters. My thought is that the player & dm will discuss the split personalities in session 0 and work out the most fitting combination and cues for changing personalities. The player would be able to make 2 character sheets, with the health roll averaged between the two (EG if one class rolls a D12 and the other a D8, this character rolls a D10). The health would remain the same when switching but the statistics would change, potentially becoming stronger and faster at a cost to intelligence and wisdom, or vice versa - it could be a barbarian who just wishes they could hit things without becoming weak and philosophical!

The key to making it balanced, for me, is that this can't be up to the player to decide when they switch, they will have to discuss the idea with the DM and decide on whether it happens after they lose half their health, or if there is a potion which they have to resist (getting harder to resist each time), or if they don't know what causes it but the DM knows it's if someone mentions any species of fish in their earshot it causes them to flip. Maybe they flip in the presence of Goblins due to their lengthy imprisonment, during which they couldn't cast spells but learnt to become stealthy like a rogue. after rehab, they sometimes revert to their alter ego in dark places.

So, my ideas aside, does anyone know of anything like this already existing?

For Bloody Nine specifically, with the damage/battle stress trigger, it sounds like you could most easily do this with a berserker with rage reskinned as the "other" personality.  

Beyond that, honestly, I think it's bit much of a big ask to create or adopt a system where the DM and Player negotiate when the DM grants the PC a personality shift with different stats and/or powers sets between personalities.  Like maybe if it was a 1 player and DM game, but for a regular table it puts additional admin overhead on the DM for one character at the expense of the rest of the party.  It's also generally a workload, albeit an enjoyable workload, for a player to keep a character consistent.  In this experiment, you're doubling the player's workload.

I mean if it's a curse or something otherwise seen as an adversity the PC and rest of the party are burdened with and have to find a way to work through, sure.  And in that case the affliction becomes a plot hook for the party to work through and maybe overcome.  But as a feature to establish a character, and distinguish them from all the other characters, I don't think I'd run with it beyond just going with a berserker and adapting a rage mechanic.  The wizard with a secret rogue in their personality doesn't work for me either.  Yes, someone can adapt in adversity, but the level of compartmentalization where the PC doesn't have control over the compartments again sounds like it puts it more to DM management than anything a player can fairly run on their own.

As far as mechanics go I think you're on your own for this. As far as the general concept goes at the obvious go to lyanthropes from the monster manual. And I would pay special attention to the "player characters as" box in the section regarding to changes in strange quirks for that kind of arrangement.

The best RAW option would be the spell Tenser's Transformation. It's 6th level, but pretty much a perfect mechanical analog for a split personality/possession. 10 minute duration, and a risk of exhaustion after the effect wears off. If you wanted to make a lower level character with this trait, you could use the Bladesinging subclass of Wizard to simulate it. It's not as dramatic as going from STR 8 to STR 18, but it provides the dramatic combat role disconnect.

Just stumbled upon this Thread. Grün Hollow ist an RPG based on DnD 5e. They introduced a new system: Transformations. Should be a good Point to start.

I did a split personality in Out of the Abyss. My fighter got permeant madness (I am not sure if it's per module or made up) - but essentially it was a voice always offering bad advice. I spoke to the DM and asked if we could flavor it as a form of demonic possession. He agreed and on my next level, I took 1 level of Warlock. So whenever I as the player wanted, I'd do a Warlock thing (mostly Eldritch blast) - but my character's main personality (the fighter) would be unaware of it during that time the Warlock side was in charge - it was a lot of fun.