Lets imagine for a second that the enclosure is a beanbag chair, for lack of abetter example. You push on the beanbag chair with 1 hand, it goes in pretty far. Push on the beanbag chair with 2 hands, and they both go in, but with equal pressure from both hands, they will go in the same distance, neither of them as far as the frist.

The box is the same way. The subs are your hands, and with them pushing in (or out) they both compress the air the same (we'd hope so ) and since neither is pushing harder than the other, they compress the air for each other, making it seem as if each sub has thier own airspace.. all the air compresses the same throught the enclosure so all subs see the same air suspension inside the box.

Hope this makes some sense