They either switch bodies, stay within the body and have a lot of inner conflict going on, or they stay around in spirit, or pass over.

There are probably other alternatives that I can't pick off the top of my head.

Oh, almost forgot merging or blending souls...but I've only seen that between two people who are in separate bodies (soulmates usually) rather than two people in the same body.

Then again, people can blend their souls with spirits, so why not another person without a body? It's more than possible in my opinion.