Hmmm... I've never considered doing a sub install like that. I've seen the same principle used for trunk installs, but never in the cabin. That should be interesting. You'd have to use a fairly low-profile sub, of course, but that'd be sweet looking.
As long as they're not firing directly at each other, it should be fine. Just angle the subs a little bit and it should sound great. And definately be liberal with the sound deadening there, because positioned like that, there is a lot of potential for panel vibration. Also, make certain you don't have any wiring running behind the sub, as it could cause problems. I know in the Mustang that Ford decided to run enough wiring behind those interior panels to strangle a grizzly with.