How to write AlternateIdentityAmnesia as a plot point without getting into split-personality cliches?

Original discussion about my story is here to avoid being locked out of the loop.

One of my characters, Katelynn "Lynn" / Jana (an important major character out of my four major characters, excluding the Big Bad) who gets Alternate Identity Amnesia. The alternate identity amnesia will be an important plot point during the story.

Katelynn is a major character, and I want to give her this (partially as a Self-Imposed Challenge for myself to write about a character this way) as something different, but equally, avoid the Alternate Identity Amnesia being a Jekyll-and-Hyde Split Personality cliche.

How could I have her as Katelynn (regular identity) and Jana (alternate identity), possibly as a split personality, yet have it creating Alternate Identity Amnesia and make their characters avoid the "good side, bad side" type of split personality?

I took (slight) inspiration from Miley Cyrus / Hannah Montana, but my character is not a No Celebrities Were Harmed version of her; the only similarity being that both sides of the split personality are good/nice, maybe Katelynn as bright Girly Girl, Jana as a Girly Girl Perky Goth-type character who's a Big Eater.

My problem is how to make this sort of character interesting and believable. I'd apprecate any advice and help on this.,