sub enclosure ?

Would it be better to have single 15" sub boxes in my 96 chrysler concorde.  Or one box dividedwith 2 subs want to know whick will hit harder.  And I may be looking to add 2 more 15's if my friend sells me his Xtant amp.

what i would do seeing that i had a 95 intrepid and damn near tore off the rear deck and trunck lid with bass is to build a box that will be vented into the car via the rear deck and seal it off from the trunk all together and yes the more solid its built the better it will sound just so you know the same system in my sig was in my intrepid and if 4 12's will do that to a rear deck and trunk i can't imagine what 4 15's will do to one just make sure you use plenty of dyna mat. one more thing theres not enough room for 4 15's vented properly in that car.1996 Blazer, 4 HX2 12's, 2 RF 800a2's, 4 6.5 MB Quarts, 1 RF 400a4, Pioneer 8400, 220 amp alternator, 0 gage main and 2 gage amp power wire. 149.1db