Well, if they can be used in a small sealed box you could just get a Q-Logic 12" woofer box or such and just use one and run it mono. I have a single 10 in a sealed box and it puts out plenty of bass while taking up very little cargo area. I also have a long wire to it so I can move it around if I need to. Usually it sits on the driver's side up against the back seat though and firing backwards towards the rear door. That's the best way.

I don't know what to tell you about the second woofer and how to handle that with your GF!

Gearhead posted the same thing with a picture while I was typing. Even the box is what I was thinking about.