1SS, welcome. The back surrounds can indeed have different "stereo" material directed to them, which makes two a better choice than one. There is a little actual 7.1 material on some discs now which have separate discrete rear channels, but a more significant consideration is that processing such as DPLIIx can direct different sounds to the two back speakers after extracting them from 2-channel source material or the surround channels in 5.1 movies.
Using two M1s(if you have lots of space behind you for them to disperse their sound)or possibly even better 2 QSs, would be definitely preferable to a VP100 back there.
