I question the Dayton's performance with only 150watts going to it.
Even with doubling the surface area, wattage wise, you are still quite low on any sub, let alone a 15" driver.
I would try and up the budget a little if you are stuck on dual subs and look another route. But for a 12x12 room, not sure bass response will be an issue that 2 are "needed"
I would prefer 1 better sub for a smaller room, than two low end subs just to say I have two.