I can think of a reason or two: With the second sub, you are not only energizing the listening room, you are energizing part of the house that contains the listening room.
08-23-07: Markphd
Interesting, but I can think of no reason in physics why it wouldn't produce the same effect if the second sub was in the same room.
In real life, you don't just *hear* explosions, you hear their effects on other things around you, you feel the explosions, and you sense the motion of other things the explosion has set in motion as well. And you don't just hear the sound, you hear the effects the sound and impact have on your immediate environment, including the space around you, which extends beyond the space of a listening room. Subs outside the listening room would imitate that phenomenum, extending the illusion that an explosion is shaking the house.