MI, you can't just tie the two neutrals, either, because the current will be divided between them, and only half of the current will return in the same cable as the hot leg.
poorboy, you're correct that a 3-wire cable with 2 switch legs and a hot (or load) wire has no neutral, but there is still an equal current in both directions -- "out and back".
We're all familiar with the requirement for the hot and neutral to be carried in the same conduit to prevent inductive heating of the metal. A more general statement of that requirement is that any current-carrying conductor has to be closely paired with another conductor carrying an equal and opposite current. The intent is to produce a net magnetic field of zero, whether the conductors are in conduit or in open space.
