[MathReferee]

Interesting situation. I am sure there is an old case play covering it, but I have not been able to find one. I have not been able to find anything in searching this forum either, but my thoughts are that you have a false multiple foul so each foul carries its own penalty and they should be awarded in the order in which they occurred. If it were me, I would:

1) Report the foul on B2
2) Report the foul on B1
3) Shoot the free throw by A1 with players on the lane and the ball is live like normal.

Since the foul by B2 occurred before the foul by B1, then that penalty must come first. Assuming team A is not in the bonus, then the penalty is a throw-in. But since the foul by B1 merits a free throw by A1, then that penalty is administered. Obviously, if Team A is in the bonus, then A2 will shoot their bonus free throws with the lane line cleared, then A1 would attempt their free throw with players on the lane.

I could understand why this would not be covered in a case play or anything as the situation would be weird. It is difficult to imagine this time sequence occurring in reality.