This is done all the time. I have the exact setup as Ken described in two of my planes. And, believe me, it will save your plane one day. I have a Venus II pattern plane, OS 1.20AX, Spektrum radio setup with five servos. I have two 6 volt battery packs, two switches, plugged into two separate channels on my receiver. As said, you can also do this with a Y cable, two into one. I also have a voltwatch hooked up to a vacant channel.

One day, as usual, I pre-flighted, checked everything, first battery A, turned it off, then battery B, then turned both on. They checked out fine pre-flight. When I was finished, and did a post flight check, I found that one of the batteries was showing down in the red on the Voltwatch, so I checked it with a loading voltmeter.. it showed about .1 volts. The other battery was fine. I replaced the battery that showed .1 volts and continued flying. Had I had a one battery setup, and that battery died in flight, well, I would no longer have a Venus II.

So, yeah, two batteries is a viable setup. It's not absolutely necessary, but is clearly viable.

CGr.