Look at 300.3(B). This section requires that all conductors of a circuit be contained in the same cable or raceway.
Using the 'hot' from a 240 volt branch circuit used for other purposes and returning on the neutral from another is bad for two reasons. One is the stray EMF you build by having the circuit conductors separated; two is the possibility of shock to some poor electrician who is trying to work on the circuit having the "pirated" neutral connection. He turns the CB "off", then checks for voltage at the receptacle box. Finding none, he proceeds to tear into the wiring, only to get caught up on the open neutral, which is carrying the return current from our "pirated" connection. Ouch!
I once got caught on the return from a 277 volt lighting circuit. Hurts for days.
