How can you have a circuit using only two of three phases?
Simple... put the circuit on a two pole breaker instead of a three pole.

I would have thought that there would be a phase 'lag' where the third phase should be peaking.
Sorry, don't know the physics behind it but it works.

. I can understand a second hot in order to provide more current through a separately fused circuit.
On the same phase? That won't work, it must be a different phase... A 208volt circuit won't have a neutal (unless of course it's for an electronic stove, washer, dryer, etc where it needs 120volt) but will be on a 2 pole breaker.