Of course there's no problem if they're on opposite phases. Question is, will it remain intact?
For example, NYC code requires #12 to be used on 15A lighting circuits. Does that make sense, when #14 is fine? The only reason I can see would be the possibility of Harry Homeowner or Harry Handyman (who it should be noted can't do any electrical work in NYC) by the substituting a 20A breaker into the lighting circuit, or a wiring change where part of the lighting circuit gets on a 20A feed.
I don't know if this is the actual reason, just my educated guess. It just doesn't make sense, if #14 can carry 15A, why is #12 mandated? (And it's not like #10 is mandated on 20A, 20A #12 is okay in NYC -- length of run aside.)
Same principle can be applied to multi-wire with shared neutral. Panel is tight on breakers, and the home owner decides to finish the basement, either himself or with a GC who does "everything". Hmmm, let me put some tandem breakers in and move this breaker (the one for the multi-wire circuit) over here (onto the other phase). Now if that multi-wire circuit is overloaded, it's possible the fire department will be stopping by.
Also, with a shared neutral, you now have 240V in a box somewhere. So let's say someone piggybacks a new 120V circuit on the two hot legs, then plugs something in this new circuit. I don't think those books at the orange box mention anything about multi-wire circuits.
I guess I should've clarified the screw-up factor. I know the folks here know all about multi-wire requirements, and would do it correctly. But how often does your original nice, neat work stay intact over the life of the installation? Something always seems to get hacked up.
Now I'm not one of those folks that believes we need to protect folks from themselves, but not sharing neutrals sure can eliminate a lot of potential future problems.
In view of the foregoing, I still say ideally it's best to run a dedicated neutral.
Just my 2 cents worth...
Joe
