If this is not 3 legs of a 3p wye it is a violation and you will be overloading the neutral.
Art 100 definition of multiwire;
Branch Circuit, Multiwire. A branch circuit that consists of two or more ungrounded conductors that have a voltage between them, and a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral or grounded conductor of the system.
