Push Pull woofer physical alignment

... just wondering whats the most appropriate here. I'm getting conflicting answers depending on whom I ask. Basically this is for a new car subwoofer I am building. The box will be sealed at first and then I may try venting. Essentially due to the contours of the car, I am able to reverse mount the front set of woofers, to do a push-pull configuration. My question is, where should the reverse mounted woofers be mounted, in relation to their planes? Obviously the easiest would be to have the edge of the frames aligned, where I'd just flip the one speakers over.

I then noticed in professional audio systems that it appears as though the middle of voice coils are aligned such as here:

[IMGDEAD]http://www.eaw.com/images/products/DC1.jpg[/IMGDEAD]
37499TAD_cinema_s.jpg



Question is, wheres the best plane to do a push-pull? Or would it not really matter?
 
Generally, the acoustic center of a drive is very close to its voice coil. Therefore, mounting your drivers so their voice coils line up vertically is probably close enough for us amateurs. The advantage in using the push-pull arrangement is that it helps to cancel 2nd order harmonics of the drivers themselves, thus producing a cleaner bass.