Well.. you're close. Two single pole breakers with a handle tie provide independent trip so a fault on one leg does not trip the other. A two pole common trip breaker disconnects both legs simultaneously on a fault. The two pole or handle tied singles are required so to disconnect both legs when servicing. When the circuit is providing line-to-line loads then the breaker has to be a two pole common trip to provide total disconnection on a fault.
Sorry if I said it wrong. I wasn't talking about any fault current tripping. A multiwire branch circuit needs a disconnecting means to disconnect all ungrounded conductors simultaneously. I merely meant that a 2-pole breaker or 2 single pole breakers with a tie handle would accomplish this. As far as my understanding, both are permissible by code.
