Enter Luigi Pirandello, who won the Nobel Prize for literature in 1934. His most famous work, Six Characters in Search of an Author, was not written to illustrate Assagiolo's theories on sub-personalities. But you know what Freud said: With every one of my theories, a poet was there first.
If you've never seen the play, a fine black-and-white film version starring John Hurt is included here via YouTube. It's preceded by an excellent 9-minute clip that sets the drama up for you and provides a bit of background on Pirandello's life and art. After you've seen the play, you might try a Google search for Roberto Assagiolo, his ideas on sub-personalities and his "psychosynthesis" approach to finding the author within. There are others far more qualified to discuss all this than I, but I couldn't let Pirandello's birthday go by without offering some small homage on this website. (The 9-minute set-up begins in frame 2 and is labeled Luigi Pirandello Part 3. It's part of a larger work, but only the relevant section about the play is included here. The play begins in Frame 3 and is labeled SIX CHARACTERS IN SEARCH OF AN AUTHOR, Part 1 of 7. The other six parts follow.)