Will two 15 inch subwoofers? Read description.?

Will two 15 inch subs fit in a trunk with the proper box dimensions? I'm afraid of getting two 15's cuz I'm afraid I won't be able to build the proper box to get the most SPL out of them. So will two 15's fit with the proper box dimension? Just in particular for any 15 inch sub, I'm not sure if what sub I'm exactly going to get yet, so I cant give you the specs of the sub.