I would stick with 1 subwoofer. For a couple reasons. 1. you can still hit decibels just as high as 2 subwoofers. 2. right under cockpit, kept out of the way.
Honestly, if your choosing between 1 or 2 subwoofers, go with 1. and get a good one. PLus i prefer class-d mono amplifiers.
saves space. sounds jsut as good.
I cant think of one good spot for 2 subwoofers. Unless it is "truely" custom set-up, you can figure it out. but if it is just one big box......g'luck.