I belive that there is only implication that this is not allowed, but no actual code to back it up.
The physics certainly works.
Bob pointed out the explicitly permission for a common neutral in 225.7 for _lighting_ equipment installed _outdoors_.
215.4 explicitly permits feeders with common neutral. You are explicitly permitted to install 6 ungrounded conductors (for 2 separate three phase feeds) with a _single_ suitably sized common neutral. If I read this correctly (and I've not delved too deeply), I don't even see a requirement that these feeders originate from the same panelboard or even the same system. Imagine the possibilities for ugliness
210.4 permits multiwire branch circuits, and article 100 has a definition (Branch Circuit, Multiwire) which specifies that the ungrounded conductors in a multiwire branch circuit have a voltage between them.
300.3 has explicit permission for the shared neutral bus in a gutter attached to the panelboard.
One could argue that the lack of explicit permission for common neutral branch circuits in article 210 implies that they are not permitted,...but IMHO this is a rather weak argument that would not stand up legally. It most this lack of explicit permission or prohibition suggests that this is an extremely uncommon practise.
-Jon