The only reason I would have one 12' is for sound quality. One 12 adds or 10 adds just the right amount of bass to the car/truck to make you enjoy what you are listening too. But if you want to be heard down the street, go with a ported box, and add another 12'. If you just want enough to hear it outside the truck you can make a ported box for that one 12.
Your set up sounds like it was designed for sound quality not quantity. I bet it sounds shit hot inside the truck and that is all you need.