Using two battery packs on your receiver is a great way to add redundancy to your control system. It's a shame to have your pride and joy fly away due to a bad cell. All that is required is two battery packs, two switch harnesses and a Y splitter. The connections should be as follows:

I have heard people say that you can not hook to batteries in parallel. The argument is that as one battery fails the other pack will charge it and loose all its capacity so they must be isolated by diodes. This is simply not the case. NiCad batteries take a lot of voltage to charge them up. Charging a 4.8V pack takes a minimum of 5.6V to initiate the charge process. I fully charged pack can not supply this much voltage so one pack will never charge the other. However, you can not charge 2 packs in parallel. This is why 2 switch harnesses are required. Each pack must be charged on its own circuit. The battery packs do not even have to be the same capacity but they must have the same number of cells.