Tom, I'll try the clarification. When two subs are used, output is increased regardless of whether they're placed close together or widely separated. When widely separated the increase is the same 3dB experienced when any two speakers are playing the identical signal. When the two sub drivers are located within about 1/4 wavelength of each other(e.g. about 3 1/2 feet at 80Hz)they reinforce each other and can provide as much as an additional 3dB increase, for a total increase of as much as 6dB.

The advantage of the widely separate location, although it doesn't provide as much increase in level, is that it can help in counteracting troublesome room modes. The basic positioning, as discussed more fully by Dr. Toole and his colleagues at Harman in the paper which has been referred to here several times, is to place the subs in the middle of opposite walls(either front and back or to each side). This would be especially useful in a room that's square or nearly so.

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Enjoy the music, not the equipment.