I cannot find anything that explicitly covers applying 3-4-7 in a double foul (or offsetting dead ball fouls) scenario. There is a case play (3.4.7.C) that covers a live-ball/dead-ball situation where the penalties are applied in the order of occurrence and therefore 3-4-7 options are given in the order of occurrence as well.
Extrapolating that scenario slightly, I would think both teams are offended and therefore both teams could have the option to delay the clock. Since you can only decline the option and not force the clock to start on the ready, if one team wants the clock started on the snap then the other team's choice is moot.
If it seems obvious that A fouled intentionally to try and force that scenario (which is a bit of a gamble, since you'd need B to retaliate), I'd probably invoke 3-4-6 and overrule any 3-4-7 decision.