I could use some help in designing a switch that will control two thermostats with a switch. The thermostat's send a 24v signal to an air conditioning unit to turn it on and off. I want to put a relay between the two thermostats controlling wire so that the air conditioners CANNOT run at the same time. The circuit should let the first thermostat that wants to run send its signal to its air conditioner while breaking the circuit from the second thermostat to the second AC until the first one calls for stopping the AC (i.e., the thermostat will break the 24 volt lead when it reaches the desired temp). When this happens the other unit could then send its signal to the ac if necessary. The problem is that only one should be allowed to run at a time but once it gives up control either unit could take control. Originally I thought of just wiring the 2nd thermostat lead to the relay and if it wanted to operate it would energize the relay and take control but that would place it in a master position and I don't want to stop the working a/c until it gives up control. Again I want either t/s to be able to request control, and the first one to request control keeps it until it gives up control. Any ideas?