Looking at this electricity pole near Oxford, it occurred to me that the heavy top wires pull in opposite directions. They are the same height so they would put the same turning force or moment on the pole (taking the base of the pole as the pivot point). A single wire would rely on the ground exerting an equal and opposite counter moment to keep it upright. There will be a lot less of the pole below the ground which means that the force exerted by the ground would have to be much bigger than that by the wire to make up for the lack of distance. Simpler to have the forces on the wires balance.
