Bruce, it depends how you'd use the second mains. A receiver that has A and B main terminals is just a parallel connection to the same amplifier channel. If you'd run a parallel set of speaker wires from the front terminals to the second speakers, that would be the same electrically. Of course without an A/B switch the second speakers would always be on at the same time.
Another possibility if you wouldn't use your sixth and seventh amp channels for back surround speakers is to instead use them in a zone 2 mode for the second set of main speakers. The manual explains how to do this.
