After buying a new Panny BD10A DVD player, i realize that very few blue ray movies have the 7.1 channel active using the rca phone jacks. So im wondering if i should parallel my rearmost 7.1 pair with the 5.1 rear surrounds. My amp is stable to 4 ohms (Onkyo 701) but i also have a secondary amp which i can also drive at the preamp level. Any pros and cons, thanks?

you are effectively turning it into a 5.1 system, then?

I once did two rear speakers in series on the surround back channel on a 6.1 receiver (of course this raised the overall impedance and did not stress out the receiver) but this didn't take away from anything.

It seems what you are planning takes out the 2 surround back channels altogether; while this would be ok - it also means that PLIIx processing would not be available to you. Don't even think of running cables to the speakers from two separate amps.