The important issue to remember when running multiple circuits on the same neutral is to phase them properly. There is nothing wrong in running 12-3 to say, the refrigerator and branching off of it to a small appliance circuit provided you pigtail the neutrals at the refrigerator outlet and say, use the red for the refrigerator and the black (jointed in the box) to the other circuit. The neutral, being on different phases, will never draw more than the highest amperage of any particular phase. The overcurrent protection can be put on either a double pole 20 with a common trip or two single pole 20's with a "trip bar" that will trip both breakers if one trips.
