"Section 200.8 Neutral Conductors
Proposal 5-49 Log No. 3644
(New) Neutral conductors shall not be permitted to be used for more than one multiwire branch circuit or for more than one set of un-grounded feeder conductors unless specifically permitted elsewhere in this Code.
This new rule generally prohibits neutrals from being used for more than one multiwire branch circuit or one set of feeder conduc-tors. No rules actually and specifically prohibited the use of common neutral conductors in other cases."
Remember, this has not become code yet, but may be close to being so.