Another Play from Pit-SD
 

Polamalu intercepts the ball. During the run-back PIT is called for Illegal Block in the back. SD is also called for a personal foul.

The referees put the ball back at the spot of the interception...

Is this correct?

It sounds like it would be for the NFL. They don't want to wipe out plays completely when possible. We would let B decline A's PF and keep the ball after enforcement. The NFL actually enforced both of these penalties by stating they offset after the change of possession.

There are other situations where, even though there is a foul on both teams, a 5 yard penalty is declined by rule because the 15 yarder must be enforced. In our game we would offset but they want the big penalties enforced.

If you put our penalty enforcement rules on a flow chart it would look like a stick with one branch. An NFL chart would look like two huge trees grew together.

Here is a rule reference for the enforcement on this play:
Rule 14, Section 2, Article 3- If a double foul occurs after a change of possession, the team in possession retains the ball at the spot where the team in possession's foul occurred so long as that spot is not in advance of the dead ball spot. In that event, ball is spotted at dead ball spot.

Ok, didn't realize it was an NFL rule... all other codes would replay the down (or B would have to decline the PF to keep the ball).

And this is why NFL officials bag the spot of an interception.