Why don't you integrate? Isn't that the mainstream way of dealing with multiple personalities?
Not all multiples consider their situation undesirable or to be of severity to necessitate psychological treatment. While it is acceptable in mainstream society to be one person per body, not all people are created as carbon copies of one another, nor is it necessarily desirable to move in that direction. Some multiples accept the way they are as a unique aspect of their being. As long as they are able to cope and function in society, remain law-abiding citizens, and bring home the bacon, it becomes arguable whether it matters if they are one person in the body or many people in the body. If everyone were expected to be a multiple, it probably wouldn't be desirable for singletons to seek out random spirits to invite into their head. For many multiples, the people who share their headspace are family and friends, and have worked very hard on upkeeping their intrapersonal relationships. They have no desire to integrate with their friends and family, and love them the way they are. Some multiples couldn't even figure out who the person would be to integrate into.
Some multiples consider attempting to integrate an act of murder/suicide, as in at least some cases, one of the people/personalities is subsumed by another. [Sounds cannibalistic. Eeeew. -- XES]
There are at a few examples where an integration has occured and the whole is less than the sum of the parts. That's a legitimate fear.
(There probably needs to be something on how you deal with the integration of of multiple friend/lover/etc. That can be particularly difficult to deal with, whether the other person is a singleton or multiple).
See also IntegrationLoss.