You can indeed connect multiple channels on the 3808 to one pair of speakers. I believe that you can actually connect as many as three channels to the fronts for example by configuring all channels as fronts in the menu.
Randy is correct that this doesn't buy anyone anything but the technical explanation is not correct. The receiver, when connected this way, is actually able to provide more power into a single channel. The problem is that the majority of speakers simply cannot use this power (including the 80s).
The reason for this is because the maximum voltage of each amplifier cannot be exceeded (ie. fire and smoke). Since all channels are connected in parallel, your receiver's power supply can indeed dump three times the current into each channel but the rail voltage of each channel is not permitted to exceed its rated value. You'd have to find a very, very low impedance speaker to take advantage of the extra current. Having the current doesn't do any good if your amplifier channels experience voltage collapse.
The best explanation I can offer as to why this is made available is so that people can double up on front speakers if they wish. Doubling up on fronts lowers the over-all speaker system impedance thereby requiring more current. So, if you want to take advantage of this method, double or triple the number of M80s for the fronts
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