If one battery has a shorted cell and you do not have diodes between the two batteries, then of course you will have a failure mode, but it is not too bad if you have two 5 cell batteries. Believe it or not, I have tried running a 4 cell and a five cell w/o diodes into a single rx with no problems. Try it as an experiment before you respond to my comments...I ran a test on a plane on the ground with this set up with a prop engine running, had no range problem or servo problem. The discharge between batteries was very slow. This was just an experiment.

Two similar batteries w/o switches will give you the best reliability and least expense and weight. I also like Nicads due to their cost and toughness even thu they are a few oz heavier than the less desirable nimh...