As Kevin has noted, it is best to put two rear surround speakers up, but it looks like you've already committed to that and are wanting to run both of them in parallel from a single amp channel. I'm currently running both rear surround channels off one amp channel, but only because the right channel of my old Parasound is messed up. It can be done, but you'll want to make sure that the amp can handle the load of both speakers on one channel (which for rear surrounds and a 770 is not likely to be a problem) and you'll need to remember that any future upgrade to a discrete seven-channel format like Logic 7 or Pro Logic IIx will require you to separate the two speakers back out.

What sort of unpowered sub would you be driving with the extra channel of the 770? Powered subs are certainly more common, but there are still a good number of passive subs on the market. The 770 is a pretty beefy amp that could comfortably drive some passive subs, but the amps most often used with large passive subs these days (like the big CS subs from SVS) tend to like amps that are more purpose-built for sub duty. If you are still shopping for a sub, powered subs like Outlaw's LFM-1 (which has a 325W amp) do offer a lot of value. If you already have a passive sub and the 770 has more power to give it than the existing amp driving it, then your scheme could be worth experimenting with.