There's several factors to consider, the 2 12's will have more surface area to move air with than the single 15 as well as two motors running to dissipate heat build up. If you were to give the 12's their proper space, and proper power (not even sure what L7's call for wattage wise, I know nothing of Kicker) I would imagine the 2 12's would bump a good bit louder.
But again, not terribly familiar with Kicker.