I'm not sure if this would work in a plane as a back-up. But in RVs and boats in order to use 2 different batteries for instance a car battery and a deep cycle battery, A simple cross-over circuit is used that can be purchased at any RV or marine store. The cross-over is pretty much just a high current rectifier bridge that allows current to travel only in one direction.
"In designing my back-up system I took care to completely isolate both systems . . . except for the occasions when I might want to connect them . . . which would be to charge them concurrently, either in the hanger or in flight."
