I think your idea for the dual woofers would be a great one, especially the MTM config. I considered this myself, but I came to the conclusion that four smaller speakers would be better then putting everything into two giant ones. Mine are for medium to large sized dances though, so your situation probably doesn't apply like mine did. The two of them in parallel should net you 3 db more sensitivity, and 6 db more if the amp will double it's power output with the new load. They will be large cabinets, but no larger then a pair of single woofer 10 pi's.

I tuned my Kappas to 40 Hz in a 5 cuft ported box. The result was very flat response down to 40 Hz, with an f3 of 36-37 Hz. Anything lower will get you flat, unresponsive bass and anything higher will start to be boomy and peaky.

Good luck!

Adam