I actually have the blue-sea systems "add a battery" kit. It includes a red rotary 3 position switch. The positions are "Off", "On", and "1+2", the latter being labelled Emergency. It bridges the starting batteries and the house batteries together.
The other component of the add-a-battery system is the isolating/combiner. When either side of the combiner is showing above 13.x volts, it combines the house and the vehicle systems so that whatever your charging source (engine, when underway, or shore/gen power when moored) is charging the whole vehicle.
Great system. I am super impressed with Blue Sea products. (I also have thier fuse block and one of their 225A fuses)