split personality

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Multiple personality disorder. No longer in scientific use.
• noun Schizophrenia. No longer in scientific use.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun psychology the supposed psychological syndrome in which two or more personalities exist in the same person

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a relatively rare dissociative disorder in which the usual integrity of the personality breaks down and two or more independent personalities emerge