[AceX]

What you can do is create a special box (can't remember the name of it right now, where you build individual boxes for each 6x9, but then link them via a large PVC pipe and cut a port into it in the middle.

Basically it would look like a large pipe with the speaker boxes as book ends. You would need to put the two speakers as far apart as possible and cut the hole for the port in the dead center of the pipe. This configuration can give two 6x9's relatively the same output as a 10-12" sub.