I have a 25 year old gas furnace without A/C. I had a 2 wire basic thermostat that I have replaced the wiring with 5 wires and also a new Nest 3rd generation thermostat. On the wall beside the thermostat is a humidifier control to run the fan that connects to terminal "G" in furnace. The new thermostat also requires terminal "G" for the furnace... Is it possible to have both the nest and humidifier control connected to the "G" terminal in the furnace or will something go up in smoke? The nest does not automatically run the fan when humidity increases, it only has a setting for a timed control.
As long as both thermostats are also using the same R wire, that should be fine. Internally, the only thing the thermostat does is connect R to G with a switch. Have you looked at all the documentation for the Nest? There may be a different, "best practice" way to connect the humidifier control.