How do I wire a switch between two 24V heating only thermostats to switch between thermostats?
I think you need a (dpdt) double pole double throw switch g is the fan, and w the heat both are connected to R 24v using the common of the switch back to the furnace on one side connect thermostat w on the other g.  Do the same with the second thermostat and when you flip the switch it will change the input from one thermostat to the other. Trying to make it clearer Many switches will have the marking NC and NO and common. Stat 1 to NC contacts and stat 2 to NO contacts and the common goes back to the furnace.