I've not used the emergency
"combine" switch feature, but I am not sure if I turn the start batt off the combine switch simply connects the house to the start circuit. If so then it wouldn't drag the house down.
In fact I am not even sure if that switch parallels the house and start banks. so that it becomes a "both" switch.
just looked at the wiring
diagram and see that you can take the start bank out of the circuit and use the house alone to start the engine