The key word in 300-3 (b), to me, is "circuit". Not to be confused with "branch circuit", which is defined in Art. 100. Rather, the current path to and from a load.
The multi-function unit is built with individual loads each with their own neutral and hot. The installer is one who has to make sense of the multiple neutrals.
When I'm installing an unfamiliar multi-function device that I'm running more than one cable to, the image I work with is that the net magnetic field about any individual cable must sum to zero under all normal load conditions. Any switch leg current in a cable must return only in the neutral in that same cable for their individual magnetic fields to cancel.
It's interesting to me that "circuit" by itself is not defined.
Tom, I've ordered in NM-B 4 wG. Got a spool of 14 & 12 that I keep in the shop for special occasions. Crescent Electric brought it in from Montana or Wyoming, I forget which.
Al
