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.
At lower levels, this problem was quickly resolved by a simple sleep spell. By the time sleep fell off in terms of usefulness, character development had left my evil side with a sort of begrudging respect for my allies. (though I still didn't care if they just so happened to be within splash damage range) By that point it was more of a mixed bag type of deal. The evil form was undeniably effective at nuking whatever threatened us, but couldn't be trusted to handle delicate negotiations. The good side would outright refuse to help in battle, but was a superb utility party member. Because of how the mechanics worked, being evil was only a temporary problem that would resolve itself as soon as I take a rest, which obviously I would need to do to get my health and spell slots back. So to answer your question: yes at first, but eventually no.