As far as I can tell, there's no case play that covers this specific scenario -- a couple covering if B fouls twice, but not one for each. However, I agree with PABJNR that once the decision is made, it's been made.
I do not agree that it's similar to the fair catch rule, as that relates to how the ball is put in play (snap or free kick), not with the acceptance or declination of a penalty. I would argue that the absolute point of no return is the ball being marked ready for play (although, it's probably much before that).
We've probably all had scenarios where a coach didn't process the results of the play and first reaction was to accept the holding foul on 3rd and forever, then realize it'd be 4th and forever if he declines it and change his mind. As long as the penalty hadn't been marked off and ball marked ready for play, I have no problems with him changing his mind. However, we wouldn't let him let the next play line up, call a timeout and then decide to change his mind.