How does a street light work with two hot legs?

A load using two hots instead of a hot and a neutral only means the voltage applied is additive. In a three phase power system, instead of using one hot and one neutral (yielding 277 volts), they use two hots (yielding 480 volts). This will lower the current draw of each light and therefore increase the number of lights that can be put on one circuit. Electricity doesn't need a neutral to return to the source. It can travel back on another 'hot', as well.