I have a pair of heat pumps in a dual zone setup, each of them has a backup electric furnace as part of its indoor air handler. Newer ones can keep running without that even into quite cold areas.
The thermostat just sees the resistive heaters as another phase, so I have three phases, and if it doesn't see a temp rise within a certain time of calling for phase 1/2, then it goes to phase 3. Mine also has support for an external temperature probe, that can skip 1/2 if it is already too cold.
I also have other ways to make heat if I have a prolonged electrical outage, but outside of maintenance, I've not used that.
The thermostat just sees the resistive heaters as another phase, so I have three phases, and if it doesn't see a temp rise within a certain time of calling for phase 1/2, then it goes to phase 3. Mine also has support for an external temperature probe, that can skip 1/2 if it is already too cold.
I also have other ways to make heat if I have a prolonged electrical outage, but outside of maintenance, I've not used that.