That is another good point xenonman. 7.2 processors are actually adding the 3 channels from the original content 99% of the time but it is designed to benefit the listening experience. The x.2 is so that we can have multiple subwoofers and equalization so that we can better deal with room resonances from our low frequencies not so that we can have a L/R subwoofer. This would not help as sub frequencies are omnidirectional and you shouldn't be able to place where the sound is coming from anyways. And it doesn't allow for seperating the LFE material from one to the other like you said.
What I actually have heard of people doing and what my local custom installer recommends is having 1 or 2 large 15 - 18" subs with one of the LFE channels produce the very lows and 1 or 2 10 - 12" subs with the other LFE channel to make a better blend into the full range audio spectrum. I'm sure there is actually tweaking you can do to the actual sub eq's and inside the processor setting but I wouldn't try and seperate the material too much if it were me.