Doc
Look in the begining of the NEC Article 100 and look up the definition of "Branch Circuit, Multiwire." It states that a multiwire circuit consist of two or more ungrounded (hot wires) with a potential between them. this means hots that are from diffrent phases, so a circuit with both hot wires from the same phase is against the code and unsafe as Sparkey said. The neutral will carry the sum of the two circuits rather than the imbalance. In a perfect world you could have a multiwire circuit where each phase had the exact same load on it and the neutral can be lifted (remember this is just for an example.) The 110V circuits will continue to operate as if nothing is wrong. The neutral is not being used because the hots are in balance. In reality there is almost always some load on the neutral. Say one hot has 10A and the other has 14A on it. The neutral will carry 4A, the imbalance.
Dave T.
