We have a propane furnace and a wood furnace that we want to run off the same thermostat. How can we hook them up so that the wood furnace will run and the propane furnace will kick in if the wood furnace goes out or isn't providing enough heat?

Sure, you will need a relay or contactor, and a heat sensor in the wood furnace, so if theis is not enough heat on wood furnace sensor, relay drops out and connects the propane furnace.