ABANDONED. Boy with Mulitiple Personalities

[Celestasaurus]

It's Young Adult fiction, and I read it a few years ago. I'm not sure on the date.

The main character is a boy. He's a good kid. Never does anything wrong. Never gets in trouble by teachers or parents. Good.
Then something switches. He gets another personality. I think he calls it his Twin, and he's "cool." He does what he wants, gets what he wants, and just about everyone likes him and/or is his friend.

[Kate]

Could this be it?

The Switch by Anthony Horowitz:
When wealthy, spoiled, thirteen-year-old Tad Spencer wishes he were someone else, he awakens as Bob Snarby, the uncouth, impoverished son of carnival workers, and as he is drawn into a life of crime he begins to discover truths about himself and his family.

[Krista the Krazy Kataloguer]

E.L. Konigsburg wrote a YA novel called (George) about a boy who has a dual personality. It's been so long ago that I read it that I don't remember the details of the plot.

[Lobstergirl, au gratin]

Celestasaurus left the group. Moving to Abandoned.