here it is straight from the nec
Line-to-Neutral Loads. Multiwire branch circuits
shall supply only line-to-neutral loads.
Exception No. 1: A multiwire branch circuit that supplies
only one utilization equipment.
Exception No. 2: Where all ungrounded conductors of the
multiwire branch circuit are opened simultaneously by the
branch-circuit overcurrent device.
This section to me has been a topic of confusion more than once, but im leaning in the direction that you MAY do this as long as they are the same amperage, and have a simultaneous disconnecting device, thinking logically, if EITHER of the two legs exceed amperage, it will trip both legs, and on the other end if you need to shut of either of the circuits for service, you will be forced to shut off both to eliminate the hazards of neutrals....