Well, I have a reg cab '05, and at one point, I had a 10" and a 12" behind the seats. They were full depth, and choked for air space. So I've been down this road. Let me start by saying, you won't get them behind the pass seat. One of them will be behind the drivers seat. The there won't be enough room for them to operate correctly. If all you want is the bragging rights of "I have two 12"s", you can do it. If you want real sound, you can do it fairly well with one 12". The box I made require moving the seats forward 2 notches, but it's the price I'm willing to pay. I can fit a single full depth 12" and still have slightly more than 1 cuft air space, which while not optimum, is workable. I could probably modify my design a lil bit and get some more airspace, but I like the way the box fits. Just to give you a picture of how much difference airspace makes, I currently have a single 10" in there, running 425W rms, and it hits harder than the 10"/12" combo did at 600w.
Basically, you need to get out there, take some measurements, get some 3/4" MDF from home depot, and start building. One sheet of 3/4" mdf was enough for me to screw up a couple of times, and still build two boxes.
I got my Jesus on the dashboard, but the Devil's under my hood!