He's basially saying he made it into a push/pull setup with the two subs. This configuration is in use in some highend home theater subs.

I personally would rather have a single 12" enclosure as Randy proposed for two reasons.

One, I already have a 12" sub I can use.

Two, I don't believe in sacrificing any of the vehicle's functionality in order to have a little sound. Most of our VX here are daily workhorses, not show only trailer queens.