Use Honeywell thermostat to control 2 different heaters

You can try it...But I don't like the idea

I would prefer separate stats for each heater
Set the wall electric heater at one temp
And the L.P. furnace at another lower temp
If the wall electric isn't enough and can't keep up the furnace will come on
If shore power fails and no electric the furnace well come on

Your way you wake up cold and have to go switch the stat