Re: Question about Using a Y cable on two surround backs
When you say two outputs into one input if you are talking about the two outputs being a preamp and the one input being an amplifier, you won't damage the amp or preamp, but you would create an impedance mismatch as well as if the two outputs were not in phase a volume reduction of those frequencies. I would not recommend it.
As Loves said, there is not really much, if any, 7.1 sources, so the 6.1 system you have will provide all of the unique sounds that exists in most sources. If you have a 7 channel amplifier, you could Y the surrounds for two additional speakers, but I doubt you would notice much difference unless you have a really deep room.