that means the subs face each other and are basically mounted face-to-face. The volume of box that is needed goes down when you do this, and you have plenty of power to make them thump hard. Ask a knowledgeable local stereo shop to run the box design on their computer program to give you the volumes and/or port length(if you go ported or bandpass) you will need.
Mike
Yeah, I know mine's an N/A! :-) AIM name: my91z MSN name: my91z