Enclosure for 2 subs?

You can easily take up that much volume by adding braces, or doubling front and rear baffles, for instance.  But keep in mind that 1 cu ft per sub might even be better in your vehicle than .8.....you might just want to give it a try first.  If it's a premade box (which I imagine you're talking about), it's kinda hard to add baffles and braces.  I suggest trying it without making changes first, then remove a woofer and place a brick inside the box for experimental purposes.  A measurement of the brick will yield cu ft of displacement, and you can decide if you want to glue blocks of wood inside permanently. 

Or use pieces of 2 X 4 instead of the brick for the experiments.  I'm willing to bet you'll not hear a difference between 1.0 and 0.8 cu ft.

(0.2 cu ft = five pieces of 13" long 2 X 4 framing stud.)