Me thinks he plans to have 1, Double-Barreled Gun, and just give them both barrels every 'shot'.
So there should be no reloading problems, and no Two-Weapon penalties.
D'oh! Misread. Well, that works, then. I guess that explains where the -6 penalty is coming from, -4 from shooting both barrels and -2 from Rapid Shot, though that still 4 attacks, not 6, so you only pull the trigger twice.