Drkmts

So im building a box for inside the rear seat of a jeep tj. Im using pioneer shallow mount subwoofer. Excuse the noob question but..... Should i use 2 subs in the box where the box size is at its smallest cubic inch size or 1 sub when the box is at its largest.
So.. 2 subs at .35 cubic feet each or
1 sub at .7 cubic feet.
Sorry for the noob question, im just really not sure.
Thanks