Nothing wrong with side firing woofers, provided they are "subwoofers". I'd want to keep their top end response below 90 Hz. 80 or lower. It'd be best to use steep filters too, 18 dB or, better.

I'm building something similar right now. Just an MTM with a "subwoofer" compartment underneath. All integrated into one box.

You'll have to better explain the dual in phase opposing woofers. Not sure I follow. Though, I'm thinking box volume could get tricky if you start adding a 2nd 10" or 12" woofer in any alignment.