[travelin texans]

Unless both the units are equipped with heat pumps only one should control heat as there's only one furnace. If I were guessing the t-stat in the rear will not control the furnace. If it's a heat pump it would control that & would produce heat til the outside temps got down to 38-40 degrees then only cold air.
The front t-stat if a heat pump should operate as a heat pump til the temp inside drops 3-5 degrees below t-stat set temp then furnace should fire up. With no heat pump the furnace should start up.