It would seem like a good idea, if you have two separate circuits (no common neutral) in the same box, to have a two pole breaker. Otherwise you could in theory turn off one breaker, check the receptacle which would be dead, and then reach in to try to do work only to shock the crap out of yourself because the other one is live. I'm surprised it's not a code requirement to have all accessible wires in a box on the same circuit, be bridged, or in the very least labeled as such.

Craig

Section 210.4 of the NEC addresses multi-wire branch circuits, noting that these circuits are those where all conductors originate from the same panelboard or similar distribution equipment.
An approved handle tie applied to two single-pole breakers for such applications as shared neutral installations ensures the installation meets the requirement of section 210.4(B), “Disconnecting Means,” of NEC 2011 which states:
“(B) Disconnecting Means. Each multi-wire branch circuit shall be provided with a means that will simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates.”