The 2008 NEC clarifies that all the conductors of a multiwire branch circuit must come from the same panelboard.

The disconnecting means for a multiwire branch circuit must simultaneously disconnect all the ungrounded conductors of the circuit at the point where the branch circuit originates. For example, in the 2005 NEC the ungrounded conductors were required to be disconnected simultaneously if they supplied a multiwire branch circuit on the same yoke, like a split receptacle. Now, regardless of how they are used, all ungrounded conductors of a multiwire branch circuit must be disconnected simultaneously at the panelboard that supplies the circuit.

Single pole circuit breakers with handle ties are permitted as the disconnecting means for multiwire branch circuits. Two-pole circuit breakers are OK to disconnect the ungrounded conductors for 3-wire branch circuits, and three pole circuit breakers are allowed for 4-wire branch circuits.

If handle ties are used on single pole circuit breakers to provide the simultaneous disconnecting means for multiwire branch circuits, when one circuit trips on an overload or short circuit, the other circuit breaker(s) may not trip. The handle ties provide a simultaneous disconnecting means, not a simultaneous trip.