What is a two phase furnace?

A two phase furnace includes 2 types of heat. Usually it is composed of an electric heat pump and also a nat. gas/ propane heat source when the temperature drops below a set range. The reason they use these is because heat pumps are very efficient when there's not a large temperature change required, but they're efficiency drops dramatically when the outside temperature (or ground temperature) drops below freezing.

The only other two-phase furnace would be a two-phase electric (as opposed to a three-phase electrical furnace.) But I don't think you're asking about 2 vs 3 phase electrical furnace systems... Those are quite rare.