A friend of mine wants to upgrade his thermostats to WiFi thermostats but the thermostat wires ran to the thermostats are missing the C wire. There are 2 thermostat lines ran from 2 units to 2 different thermostats. We can run a new 18-5 wire from one of the furnaces to one of the thermostats. But we can't get the other unit open because of the way it was installed. Is it possible to split the common wire down at the thermostats and use it for both new thermostats?