[BigBoy2U]

I have dealt with commercial HVAC and set points. An example is the Honeywell Chronotherm thermostat. It is not possible to run both the AC and heat at the same time the thermostat locks it out but....it will cycle back and forth every few minutes from heat to cool if the set points are to close. IE 72 for heat and 72 for cool. I think the key to this whole thing is this....turn off one before you turn on the other. I am sure you could employ some type of three way switch that would swap the power from one thermostat to the other if you really wanted to spend the time at the fuse pannel to wire in a 3 way switch. A simple ON-OFF-ON would work to send 12v to the furnace thermostat and then 12v to the module located in the roof AC unit. But then we are adding an "ugly" switch in the wall next to the thermostats. For me I think its just a matter of keeping one off while the other is on. Worst case you run up a whopping bill for electric and burn off a bunch of propane as both units are on...LOL