For _non-metallic_ wiring systems, I do not belive that there is a _requirement_ that all conductors travel in the same raceway/cable assembly. At some point one hits performance requirements, such as having a low impedance fault current path...but you would have to have a pretty extreme situation for the inductive impedance of the loop to make a difference.
IMHO it is a very good idea to minimize loop area, for example to minimize 'emf' (no proven dangers here, but why go out of ones way to make a potential threat worse when doing things right essentially eliminates the issue). But I see no significant difference between running a _non metallic_ 14-2 and 14-3 side by side, versus running a single 14-5. The junction boxes would either need to be non-metallic, or both cables would need to enter through the same cable clamp (using a clamp suitable for two cables).
I am in total agreement that for metallic wiring systems, the inductive coupling to the conduit or cable armor is a significant problem, and that this would be a code violation.
-Jon
