Enforcement puzzle

[wisref2]

Result of the play is a touchback. If B accepts the penalty, it creates a double foul - replay the down (by rule, A has to accept the penalty against B).

There is a chance that B would decline - and this option has to be explained. Let's say A had the ball at B's 5 yard line. As B, I would decline A's foul. The ball is then placed at the 20 (A's ball after touchback), and the BIB foul moves it to the 30. First and 10 on A's 30 is a whole bunch better than repeating the down at B's 5.