Hey guys, take a minute and read 10-1-6b. I believe this is the governing rule since there is live and dead ball fouls. Another part that is relevant is this is in an area where half-the-distance enforcement could come into play.
The live ball foul cancels the score and sets up replay of the try down. The two dead ball fouls are separate actions a few seconds apart and should be enforced in the order they occurred. So mark off the LB foul from the B-3 leaving the ball at the B-18. With the UNR by B occurring next, it would be half-the-distance to the B-9 followed by the UNS on A taking the ball to the B-24 for the next snap.
If enforced this way the R will have to do some explaining to an unhappy A coach. I don't have an issue with offsetting the two DB fouls but by doing that are we REALLY following the rule? If the ball is in a position where the order of fouling team and half-the-distance enforcement is not part of the equation, then certainly offset. Modify this scenario by saying A's DB foul comes before B's. Then marking them both off puts the ball at the B-18, so offsetting is the net effect.