Two batteries, each with its own switch. I tend to use packs of very modest capacity; but that'll depend upon the loads which you expect to serve.

One more thing.

Each 6v pack should have a diode in its output. This way, if one pack fails, it cannot drain the life from the remaining good pack. The diode means that you need a separate charging jack ... since you cannot charge your battery "against" the diode. There is a small volt drop across the diode, under load, but it's not a problem.

I have used this system for near 20 years, now. Have had one pack fail, but never lost a model through battery' failure.