Tandem (Many Worlds Trilogy, Bk 1)

Sixteen-year-old Sasha Lawson has only ever known one small, ordinary life. When she was young, she loved her grandfather's stories of parallel worlds, inhabited by girls who looked like her but led totally different lives. Sasha never believed such worlds were real - until now, when she finds herself thrust into one against her will.

To prevent imminent war, Sasha must slip into the life of an alternate version of herself, a princess who has vanished on the eve of her arranged marriage. If Sasha succeeds in fooling everyone, she will be returned home; if she fails, she'll be trapped in another girl's life forever. As time runs out, Sasha finds herself torn between two worlds, two lives, and two young men vying for her love - one who knows her secret, and one who believes she's someone she's not.