I actually did this once, only the good personality was a pacifist healer who was a liability in combat due to her aforementioned pacifism and her oath to help anyone who asks for it occasionally helping our enemies, and the evil personality was a sociopathic battle hungry sorcerer who just wants to cause as much mayhem as possible.

Mechanically speaking, the evil one surfaces in high stress situations (And even then, I have to fail a con save for it to take effect) and I automatically revert to the good one upon falling asleep or otherwise losing consciousness in some way. I ran all of this by my dm to make sure it wouldn’t screw over the party too much or be too powerful. It was my favorite character thus far.