This is a series of I think 3 or 4 character bleed stories. The links to the sequels will be on the page somewhere.
"Method Acting" http://parenthetical.livejournal.com/68775.html

A oneshot, J2(Sam/Dean):
"Character Bleed" http://lazy-daze.livejournal.com/629871.html

This isn't exactly what you're looking for. But Jensen does have alternate personalities (of the characters he played in tv/movies). It's a Dollhouse AU if you've ever seen that tv show(I haven't but loved the story):
"Stained Glass Masquerade" http://splashpink.livejournal.com/199514.html

Jensen has MPD, but not of his tv/movie characters:
"The United States of Jensen" http://takobella.livejournal.com/4826.html