A1's pass is intercepted by B1 at B's 40-yard line.  During B1's return, A1 and B2 begin fighting at A's 40-yard line.  B1 returns the ball to A's 2-yard line.




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Ruling:  If B accepts the penalty for A1's foul, it creates a double foul.  B may retain possession by declining the penalty for A1's foul.  In which case, it would be B's ball at its own 45-yard line following enforcement of B's foul.  Both A1 and B2 will be disqualified for fighting. 

Comment:  Whenever both teams foul during a change of possession down and the team in final possession gets the ball free of a foul(with "Clean Hands") they have the opportunity to retain possession.  The fact that their opponent's foul occurred after the change of possession has no bearing on the enforcement.  However, when B, the "clean hands" team, retains possession by declining the opponent's foul, the penalty for B's foul may be enforced. (10.2.2B)Casebook