In freeing the Onkyo needs to drive the front speakers, there will be additional power for the remaining speakers. Since the Onkyo and most other AVRs tend to rate their power as 2-channel driven, I would assume that if all channel were driven, the RMS power would be far less. That is my rationale for considering additional amplifier.
Thanks for clarifying for me the multichannel analog inputs and the difficulty to use the second receiver as an additional amplifier. My problem is I do not know if adding an amplifier will help my 7.1 set up at all. I suspect that in movies, when playing fairly loud, the Onkyo 809, rated at 135W/channel 2 channel driven might be underpowered for my other channels. My set up is mostly Klipsch and Acoustech speakers.
Sharp 80" TV, Onkyo TX-RZ800, Klipsch speakers, SVS subwoofer, Tivo Roamio