I'll third that suggestion. I have two pattern planes with exactly that installation, using a pair of 1200 mah NiMh packs and a voltwatch. This configuration saved a plane when one of the battery packs went bad during a flight. The other pack was there and I never noticed a difference at all. Once I landed, I did the usual battery check and found one of the two to be dead, or near dead. I replaced it with a spare that I had with me, put both on the charger to get them both peaked and then went back in the air.

CGr.