there is nothing wrong with using 4.8 volt nicad batteries.....you can use 6 volt if you wish, the battery will be of bigger capacity....you just don't see 6 volt 600 mAh batteries......as stated you can run 2 switches in parallel. where voltage will stay the same but current will be additive....in other words 2 4.8 volt 600mAh batteries, in parallel will give you 4.8 volt 1200 mAh capacity.....and will take twice as long to charge fully as a single 600mAh battery. or you can run the second battery to an open connection in the receiver.....
I have never run a dual power system....I have never had a power failure.....If you charge the night before you fly for 15 to 17 hours you will be ok...remember a second battery is extra weight.....If your worried about a brown out with your 2.4 system...get the smallest capacity 6 volt battery and you will be just fine