Eyes in the Mirror by Julia Mayer
This is the story of two girls, Dee and Samara. Samara is a cutter; she’s been cutting herself for a while now. Her mother killed herself a few years ago, and her father is distant and uninvolved in her life. His focus is work and his girlfriends. Samara is looking for herself, for a way out of her pain. That’s when she meets Dee. Dee turns out to be her reflection in the mirror – literally her exact opposite.
They live in parallel universes where they can go through the mirror into each other’s worlds and also switch bodies if they want to. Dee first discovers this and enters Samara’s world to try to help her. When Samara realizes that there is a parallel world through the mirror, she is horrified. But eventually she opens up to it and the girls talk to each other all night long. They’re exactly alike but exactly opposite at the same time; true reflections of each other.
Every teen girl fantasizes about having a double and best friend rolled into one--an alter ego with whom she can trade places, allowing her to disappear. Samara is a troubled and lonely adolescent, prone to cutting, who desperately craves both intimacy and escape from her unfulfilled life...until she meets her reflection, Dee, the seeming answer to all her problems.
With dual and dueling points of view, EYES IN THE MIRROR provides a perspective on one girl’s life never before seen in YA fiction: from her own, and from her freer and wilder reflection.
Coming August 1 2011 from Sourcebooks Fire
This one sounds very dark, but also very original and just plain good. (Am I the only one who always types "alos" instead of also?)