What Should We Have Done??

[Adam]

I'm confused, did B1 and B2 both foul A1, or is there an A2 involved somehow?
If they both fouled the same player, go with one and move on.
If there are two separate incidents (B1 fouls shooting A1 and B2 fouls reboundingg A2), then you have to figure out which happened first.
Whichever happened first is the one you go with. The coach may be right, by rule, if the first foul was B1 fouling the shooter. The ball is still live on this, but given that there will be no rebound, the advantage disappears and the 2nd foul can easily be ignored.

If, on the other hand, the shooting foul is first but B2's foul is too hard to ignore; go ahead and call them both. False multiple, penalized in the order they happened. Two free throws for the shooter, and the ball OOB for the team A (or bonus free throws for A2).