If one pack dies suddenly the other will keep things going.

However I am a firm believer in keeping things simple and always went for the one switch one battery option. But at the beginning of the season I would always cycle my NIMH packs a couple of times and check the capacity before flying. Doing this over the years I found 2 transmitter packs that were failing and 1 receiver pack that was almost dead yet read normal on a voltmeter. The small NiMH packs were replaced after 3 years. [ less than 400 mah. ]