Your grounded conductor will only carry the difference in current off of your ungrounded conductors as Roger just explained. A true neutral will carry no current...I find definite overheating potential in this situation and the only code reference I could give you for my reason for thinking this is a violation is once again article 100 definition of multiwire branch circuit. This definition clearly states that each ungrounded conductor will have the same voltage to the grounded conductor, and that there will be a voltage between each of the ungrounded conductors. If you run two conductors off of the same phase (which is what you are talking of doing in a single phase system) they will not have a voltage between them and that is a violation.
