You could, but the performance of these batteries are different(Reddy batts are better quality). The better battery will be forced to make up the difference of the other pack. Are you thinking parallel or series?
With an 8s you might destroy the motor if you put them in series (so total 12 cells 14.4v. You could run them in parallel and get a lot more runtime/more torque bceause more amps..but in series the motor will (with no load) come to about 63K. With a load your still looking at over 50k which while insanely powerful could destroy the motor. Mike would be best to say on that (if you brought it from him then you'll know where you stand with warranty if you put 12 cells on).
Ive run an 8s on 12 cells before. It can handle it. You would need s serious controller if tyou want to gear it up, though. The 8s on 12 cells is pretty rediculous power.
Rtsbasic, you were considering to use NiMh/ Nicad in parallel. I was under the impression that you could only use Lipoly packs in a ?S?P config....so does it mean that you can do a parallel config with nimh packs? This config appears to be something that is not widely published.