Quote Originally Posted by jqavins View Post
For D&D: The two characters both get full shares of XP and, therefore, both advance in level at the same rate. This is hard to fluff, and depends a lot on the fluff-of-choice for experience points and character levels in the first place, but I think it's the only way to make this work. It seems unbalanced on its face, since the one person gets all the advancement of two, but since only one is ever present at a time that doesn't hurt anything; you're always playing a character of same level as the rest of the party.
I disagree that this is balanced with the rest of the party - even though they're the same level, the character is much more versatile. That's especially true if the player can choose which personality is dominant at a given time, as they'll have the option of bringing a relevant ability to bear on almost any situation. But if they don't have any control, that's going to be frustrating to both them and the rest of the party - "No, I could easily pick this lock when I'm a rogue, but I'm a cleric today. Sorry, guys." It's also harder for the GM to plan encounters if you don't know what abilities that character will have.

Also, and this is my bigger argument against this, giving one character this kind of special treatment is going to make them the center of attention far too often for my taste. That's already a big issue with this concept, because a disproportionate amount of table time is going to be taken up with this character's issues regardless of how you handle it. But I wouldn't enable it by giving them special mechanical treatment. I'd just point them at the multi-class rules and tell them that if they want to talk in a Cockney accent whenever they cast cleric spells, that's up to them. Also, that saves the GM the headache of keeping track of which character is in play at any given time and adjudicating whatever rules you come up with for switching back and forth. Basically, don't make more work for yourself on top of everything else.

Besides, I strongly suspect this is going to get old fast once it's in play. The player is going to want to use whatever abilities are most relevant to the situation at hand, and once the novelty wears off it's not going to be fun to only have half their skillset available. If they're already a multi-class character mechanically and it's just roleplaying, then you have two advantages. First, you as the GM don't have to care when they inevitably start applying skills or knowledge from the "wrong" personality to solve a problem or switching personalities whenever it's convenient. Second, if they decide to pull the plug on the whole idea, a mini-arc that unifies the personalities is easily implemented or handwaved.