I don't get that you would use electric heat over propane. Where I come from, heating with electric is far more expensive than gas.
I would forget the electric heating and use the dual either heat pump OR gas, depending on the outside temperature.

Hi,
Yes Propane and electric are about the same here right now.
The advantage to using resistance over propane is that the coil is before the resistance so I can heat the 65 degree return air to 80 or 100 with the pump and then the resistance does the rest. I cannot do that with the pump and the propane becuase the coil is after the furnace.

Never heard of doing this before. What about a separate outdoor temperature sensor that at 28 to 30 deg. switches off the electric and switches on the propane furnace? If necessary, that sensor would operate a double-throw relay. All this control would do is determine which supplementary system could work. It would not be interconnected to any other controls. Below about 28 to 30, the heat pump compressor is not helping anyway.