Thanks for the clarification. With snapmirror you should be able to perform a snapmirror resync to be able to resync from B to a common snapshot on A1. This should be your steps:
Update A -> B relationship (from B) Break A -> B relationship (from B) Release with relationship info only true on A so the snapshots stay for the resync later. Create new snapmirror A -> A1 and initialize. After transfer is done and it is idle you can break it and release with only relationship info again. At this point you should have a matching snapshot you can use on A1 to do a resync from B.
Below is some documentation about snapmirror resync: