Suddenly Twins! — John Peel

It all began the day Chrissie Scott looked into the mirror and saw somebody else's reflection. A boy who looked exactly like her. Nothing would ever be the same again...
Suddenly Twins!
Chrissie thought she had troubles -- until she fell through the mirror into her double's life. He said his name was Chris, and his world was the mirror image of hers. His world was ruled by magic -- and about to be destroyed by the sorceress Lori Lee!
Chris is desperate. He has cast a powerful spell and sees the evil plot. Chrissie is the only one he can trust. Together they must save his world before it's too late. But is Chrissie about to be lost forever in the universe beyond the mirror?