Dual Thermostats: Which wire goes where?
You should baptise 'main' and 'auxiliary' heat, so with a single stat, you can control both systems. Using common, 'CallForHeat' and 'AuxiliaryHeat'  connections.

For single generator units (Ex: only oil heat), if you have only 2 wires, just use connectors R(h) and Common (W), as Rc is used for cooling. If you have forced air, you may have also G cable (fan). Other wires are used for more advanced setups.