User blog comment:Seieireppa/God Eater:Universitas, chapter 47: Wings Towards Tomorrow (Arc 2 final chapter)/@comment-1419424-20150729023551/@comment-1419424-20150729024036

Not necessarily. With Ken, he returns to an earlier point in time to change the course of history. The result is essentially a branching pathway created from Ken's actions.

With Blake and Jack, however, time would still be able to move in a different direction, but there is no branching pathway. There'd still be only one flow of time, if that makes sense.

EDIT:  To Bal:  Not quite. They have the same power that can be used independently. That way, if one of them is incapacitated somehow, the other can still use the power (i.e. to save the one from whatever got them in the first place).