How can you have a circuit using only two of three phases? I would have thought that there would be a phase 'lag' where the third phase should be peaking. I can understand a second hot in order to provide more current through a separately fused circuit

The latter is precisely what I meant. I see a lot of that in places that have 3-phase coming in - they use one (or more) of the phases, referenced to neutral, as normal, separate circuits.