Good question, Skip1, and as author of the rule change I'll attempt to address it.
(1) Rule 9-4-8 is a live-ball foul treated as a dead-ball foul with succeeding spot enforcement with the rationale that it didn't have any impact on the outcome of the play.
(2) IF A's USC foul occurred PRIOR to B's penalty being administrated, they would cancel (I didn't want the word "offset" used ,as I felt it could be confused with offsetting live ball fouls where 3 on one team and one on the opponents would offset) .
(3) HOWEVER, IF A waited until the penalty option had been chosen by his team and administered ; it would be enforced separately.
EXAMPLES : In #2 they would cancel and you would move on to the PAT @ B's3.
In #3, if A chose B's foul on the kickoff followed by A's USC, B could tack it onto the PAT -
back to B's 18, or KO, back to original 40. A would not be allowed to change their choice.
I needed to amend my proposal, as I had originally set the benchmark for the separate enforcement as the RFP, but the issue arose : "What's going to happen if you've already enforced the original penalty when something new occurs prior to the RFP?" Yes, #3 can get complicated, but no where near the complexity of the prior rule and would guess 90+% of the occurrences would be of the #2 style.
Hope this helps.