Personally, I have run across romex being wired as two separate cicuits with the two breakers not tied together. I call them as a safety hazard, since quite often an unsuspecting individual will turn off a breaker to work on a switch, outlet, or whatever, not knowing the wire is still hot, and may inadvertantly cut the wire. To me it's a case of common sense as opposed to relying on a code requirement, which may not be valid or adequate.