So come to find out ours has the heat pump option. Appears to be just the middle unit. I would say that we'd close the middle vent and leave the ends open and the doors to the garage and bedroom open for best circulation. You set the thermostat to electric heat and men it drops to 5deg below set point the LP furnace kicks in and the heat pump turns off until set point is reached, starting the cycle again.
Someone else pointed me to
CheapHeat. Cheap heat is an electric add on to your furnace that uses the blower of the furnace. You can get equivalent heat to the furnace on LP. But now your limited to the one duct to the rear. I'm going to look into if there is a way to enlarge that tap or add another to make that a viable option as well. To increase the airflow you could close all taps in the ceiling accept for the rear and run the front unit on fan to take warm air from the front and circulate to the rear just to even temps out.
Normally we've used ceramic heaters in th past but we have two kids in the rear and ant to be careful about their usage for safety.