Wow what are these people saying..

They need to be in phase, and it doesn't matter what side the speakers are mounted. Pressure inside the box is a good thing and when he says "no longer compresses air" he isn't just talking about inside the box, this is true for outside the box too. Sound is compressed air, no longer compressing the air is no longer making sound. You want to pressurize the room. That's probably why it effects your body too.

If you wanted to put them out of phase you would have to separate the two chambers so that each speaker can pressurize when it needs. Phase does change with distance, but the frequencies are so low that they will not be out of phase being on opposite sides of the box, this would only be true for higher frequencies. I'll bet that the designer of this box put the drivers on each side of the box to limit any movement that might occur from these large diaphragms moving, if you put the drivers on the same face the box might get pushed around or vibrate distorting the "silence" of the subsonic frequencies. Honestly I would look into designing this as a ported box instead of sealed to get more decibels out of the low frequencies, that's a post for another day. If you keep it sealed though, larger the box the better the low frequency response but you put the drivers at risk, they're only made to reach 22hz or so. Also would recommend applying resin to the cone as they suggested to lower the resonant frequency.

Unless maybe you guys were thinking of reversing the polarity and playing each driver at a different frequency? You can use the frequency cancellation to your advantage, but you must separate each driver into its own chamber. Build this box with a wall in the middle and reverse the polarity as you thought. Then play each driver at different, but at their rated and loud, frequencies they will cancel out some of the waves but not all leaving you with the difference. Hopefully high decibel low frequency waves will be left. If your resonant frequency with the sealed box is 54hz that means the box will play those frequencies the loudest. So with reversed polarity try playing one speaker at 50hz while the other at 60hz hopefully giving you a 10hz overall sound. Actual results might vary, try different tones.