Subwoofer-specific frequencies - usually <120Hz - are, as the FAQ says, non-directional, not by design by in the way we hear them. The reason a single subwoofer will suit a system is that the sound cannot be accurately pinpointed directionally by the human ear, as it can with high frequencies.
The reason you would use two subwoofers in tandem - other than having them built into our main speakers - is to supplement the LF sound in a larger or less-than-optimally-configured space, but you wouldn't need,or want, to differentiate the signals in any way to do so properly.
Outlaw has an informative section on Subwoofer placement, for both single and double subs, in their LFM-1 manual
here.