Well it really doesn't true two phase power still has three conductors because you need a return unless your three phase is a wye connection so it would save a wire.

You can use two phase in place of three phase but often it won't run as efficiently or even hot. That is bad because in the long term the trade in efficiency for cost of a conductor probably works against you.

To my knowledge a true two phase system has only ~87% of the line voltage so thats not possible. Maybe you are talking about split phase power? Split phase power works by sending two live wires, one the base voltage, and one that has two times the base voltage. Then the connection can have the base voltage and 2times the base voltage like you state.

The split phase seems the most likely.

http://en.wikipedia.org/wiki/Split_phase