Did you possibly leave the plane switched on while you were chasing the horses? It's odd for a pack to drop that much voltage unless it's left on or there is a short somewhere.

You can setup dual battery systems without too much trouble. Although I've never set up something like this on anything 60 and smaller because it's sometimes hard to find room for a second battery and the added weight isn't worth it. But you can run two batteries through two separate switches. Just plug them into two different channels in the receiver. This way a battery failure isn't a killer for the plane.

Ken