Peter,
I'm no NEC expert as you know, but I would think that 310.4 covers this.
310.4 Conductors in Parallel. Aluminum, copper-clad aluminum, or copper conductors of size 1/0 AWG and larger, comprising each phase, neutral, or grounded circuit conductor, shall be permitted to be connected in parallel (electrically joined at both ends to form a single conductor).
That doesn't actually state explicity that #14 or #12 conductors must not be paralleled, but it sure seems to imply it, and I get the impression that that's how most everybody here interprets it. None of the exceptions to 310.4 would apply here -- at least not that I can see.
Paralleling aside, I know for certain that theree's something in there about not cross-connecting neutrals from different branch circuits (just as there is in British "code"), but I can't quote a reference on that one. I looked in 210 section I (Branch Circuit General Provisions), but I couldn't see anything.
