I managed to squeeze my large Orion HCCA 250 Amp and two twelves in the rear door. IIRC, each sub required a minimum of 1 cubic foot sealed air, and I squeezed just a little over that. I used some stuffing material inside to help increase the perceived volume, and my subs sound really nice. However, if I had to do it over again, I might consider mounting the amp elsewhere to get some more volume for each of the subs, or even using just one sub. I believe those subs can be mounted is as much as 3 cubic feet of air for competition grade systems.

I don't know what the volume requirement is for your JL 12, but you should easily be able to get about 2 cubic feet, which is enough to make most subs sound incredible.