na you will only add i think 3 db for the two of them i suggest upgrading your subs cuz i said it in an other tread that rockford p2 don get too loud so try to buy others subs if ur low on cash buy a pair or cvr's or 1 18 inch re audio sx it will kill your current subs in the proper box,
if you dont want to upgrade ur subs try a better box if ur using sealed try ported and it will be louder 3 db louder but u have to tuned correctly
if someone is buying rockford subs, should buy the titanium ones "i think they're from titanium", and for the price, there are better options, that's one of the problems of rockford subs
the more subs you add, the more complicated it'll get and the more SQ you will lose.
if you have 2 12's and you want something louder, i'd look into getting 2 15's of the same kind if you really like the 12's you have, or getting 2 better 12's of a different brand.
