Robert, I haven't really given this a lot of attention, but the consensus appears to be that it isn't a significant problem. For example, Tom Nousaine, in an article in the January, 1995, Stereo Review(apparently not available online, but some libraries have the back issues), discusses optimizing bass reproduction. Among other topics he discusses "stereo bass" and describes blind listening tests he conducted in which the listeners couldn't distinguish between the stereo and mono options when the crossover was at 80Hz. Keep in mind that the harmonics of even bass tones with a fundamental around 50Hz or so are played through the main speakers primarily, not the sub, and it's these harmonics which allow a string bass, etc., playing a low note to be localized although the fundamental itself is low enough in frequency to be omnidirectional.
Sure, you can experiment with two subs along the lines that you suggest, but there doesn't appear to be any real difference in what you term an "other solution" from your preceding one. Running parallel sets of wires to mains and subs isn't any different than wiring to the subs and straight through them to the main speakers(if that's what your first suggestion is). The sub's internal crossover just rolls off the sub above the selected frequency; it has no effect on the mains, which continue to run full-range.
