We have two heat pumps with heat strips and two gas furnaces.
We enter the coach and it is cold. Turn on heat pumps. The gas furnaces kick in until it gets close to heat setting and the heat pumps then take over.If during the night it gets below about 40 degrees, I think or the heat pumps stop putting out enough heat the gas furnaces take over again.
If we desire we can select gas heat and it stays on gas or we can select zone one, the rear unit to be electric heat pump and zone 2 or front to be gas or vice versa. Our front unit is going to be replaced. it has bad valve. I have seen in a cold rain a lot of ice on the outside of the front heat pump, when left on electric. So I'm not sure about parameters on how they interact.