Piddler

I'm currently putting together the most expensive plane the I have ever built. I'm thinking about using a dual battery setup. I want to make sure that I understand how to do this. I currently have the single battery setup installed. I have one channel open on my receiver. From what I have read, I can hook another switch and battery up using the open channel. Will this provide an alternate power source if one battery goes bad? I plan on using 2 6v 1100 mAh batteries. Will both batteries decrease voltage at the same rate, but just slower than the single set up?

Thanks in advance,

Brad