This is an excellent question and I've had to deal with it before. Someone wanted to run a multiwire branch circuit as Don discribed, for lights and plugs. In addition he wanted to run a circuit for a welder. I told him he needed to have a sub-panel and he had a fit. He ultimately put in a sub-panel but he was not happy. The multiwire branch circuit was one circuit but adding the welder was the second circuit.
