Parallelogram (Book 2: Caught in the Parallel)
Audie managed to bend the laws of physics to save her parallel self from death, but now she's trapped in the wrong universe, with no sign that her scheme to save Halli actually worked.
Until she can figure out what went wrong, Audie has to pretend to be Halli--and fool everyone from Halli's parents to the guy who loves her. But what about Audie's own life? And the guy she can't forget?