Placing them on opposite poles is stupid easy but that doesn't help you.
You can't gang two cables to support the same load. It's not allowed and it probably won't work.
As an aside, it's a little weird to have two parallel cables to the same outbuilding. This, also, is not allowed unless the circuits are of different voltages or differently controlled (e.g. A switched circuit for a light). This prohibition may be your salvation: If both cables are /2, their only options are 120V or 240V, so there'd need to be one of each.
So that's the answer, rewire one of the circuits to be a 240V circuit.
If you need to switch a light, then you either use wireless smart switches of some kind, or fit motion sensors, or fit efficient LEDs and just leave them on 24x7.