man, with one 15 x.x.x. .. you can bearly hear yourself talk. with 2 you are obveusly showin off your system, you cant really hear any better or less or w/e u wanna call it with another 15, it will drown alot more noise tho, :P. Id stick to a 15 unless i wanted to buy a new set of windows also lol :P
Ya 2 15's has more surface areia resulting in more base. On the other hand, with 1 15 x.x.x it will be loud, but with to you will have to be a real bass fanatic!:-)