Disregarding the NEC violation that would be involved ... if you (as an electrician or inspector) encountered a case where some 14-4 or 12-4 cable (white, black, red, blue) was used, with the blue wire clearly and permanently marked white and also obviously distinct from the original white wire, as a two circuit cable supplying split duplex receptacles, would you considered that to be unsafe in some way?
Since it is electrically no different than using 14-2-2 or 12-2-2 cable, the only safety issues I could imagine would be that this could confuse someone and they rewire things wrong, and do something like putting 240 volts on a 120 volt outlet (by moving the blue-marked-white wire from the neutral bus to a breaker pole opposite of the corresponding red wire). If this is done only in homes (not business or industrial) that only have single phase power (or maybe 2 phases of a 3 phase system), would there still be any significant confusion risk?
How would you wire a split duplex receptacle, with the requirement of not sharing the neutral for compatibility with AFCI, if you only had available 14-2 and 12-2 cable? Would you connect the ground wire in both cables to the one single yoke duplex receptacle (and the box if metallic)? An alternative to the question would involve separate receptacles, but a common metallic box.
