Dunno if this is a GQ or a CS.
QB drops back and throws long. As the (catachable) ball is in the air, and after both DB and WR are 15 yards downfield, the DB clearly commits pass interference by punching the WR in the stomach. * Subsequently and seperately*, and with the ball still in the air, the WR commits pass interference by kneeing the DB in the crotch. In the melee, the ball falls incomplete.
Ruling on the PI please, differentiating HS/college/pro as needed.
Offsetting penalties; replay the down. I don’t know if high school is different from college, though.
Oh, and I don’t know if you’d be concerned about this, but the acts of punching and kneeing generally carry 15-yarders and possible ejection if they’re flagrant. I think the enforcement would be the same regardless of whether they called personal fouls or interference, though.
Well, I knew that; I was being hyperbolic.
My question really boiled down to whether or not the penalties would offset if one clearly happened first.
Yeah, I figured you knew, but what the hell.
As long as both fouls take place during the same down and without a change of possession, while the ball is in play, they offset, with the exception that a 5-yard penalty doesn’t offset a 15-yarder. In that case, I think, but I’m not sure, that the 5-yarder doesn’t even count. A quick Google seems to bear that out but I don’t see anything authoritative, so take that with a grain of salt. I mean, more salt than that which should accompany anything I say.
NCAA: They would offset and replay the down. In this particular case, both players would be ejected.
Also, what Jimmy Chitwood says about 5-for-15 only applies in the NFL. In college ball (and high school, I think), live ball fouls by both teams will offset, regardless of the number of fouls or their yardage (an illegal formation could offset both a 15-yard facemask and pass interference on the same down).
“…the WR commits pass interference by kneeing the DB in the crotch.”
“…live ball fouls…”
(boyish giggling) 
Hey, just wait until the tight end goes in motion.