A thermostat is basically a temperature controlled switch. To turn on a heating device, all you need is to connect the two thermostat wires together. A simple single-pole 24vac relay will be fine. Basically, connect the relay with the two leads of the thermostat from one heating system you want to control on the common and normally open sides fo the relay, and the coil wires connected to the common and output of the thermostat from the other. Then, when one system's thermostat turns on, it will energize the relay, emulating the thermostat of the other furnace.
If you search on isolation relay, you should get some examples.