Actually the sub-satellite concept is quite old. I have an issue of "High Fidelity" magazine from 1952 (!) that has a lengthy article about how to do a sub - sat conversion.

About more than one sub: I would add that not only are two subs better than one (for control of standing waves and extension/SPL ability) but STEREO subs are very beneficial, especially when listening to music from high quality sources. The "ambience" that exists in the low frequency region is more realistic when reproduced in stereo, than mono. Also, it is preferrable to have low frequencies mixing acoustically rather than electronically.