The Twinning Project — Robert Lipsyte

Tom is a smart, talented loner with a chip on his shoulder and a big secret: an imaginary twin on another planet. Eddie is Tom's opposite, a friendly, athletic kid who always looks on the good side. Tom worries sometimes: does confiding in Eddie mean he's nuts? The truth is even crazier than that. Eddie and his planet are just as real as Tom and his Earth, but fifty-some years in the past. And the twins are caught up in an alien master plan that might just mean Earth -- both Earths -- will be destroyed. Switching places and identities, "slipping" between planets and across decades, a desperate escape, and the unraveling of deeper secrets leave Tom and Eddie aware of the danger they're facing and the tools they can use to overcome it.