Multiwire branch circuit means of disconnect
The "means of disconnect" requirement for multi-wire branch circuits (NEC 210.4b) simply requires that there be a means of disconnect, and it disconnect all legs at once.  Usually, that is simply the 2-pole (240v) circuit breaker in the panel, and that is fine; that completes the requirement.  

Unrelated to the MWBC is a separate requirement that power entering an outbuilding.  It must have a disconnecting means of some kind (NEC 225.31) near where the power enters, inside or out (225.32). That's where you use a disconnect switch like the above.  There is no need to disconnect the neutral. 

I would point out that if you're doing a sub-panel, simply use a panel that has a "main" breaker and that's your shut-off switch. It's legit to have a 30A run to a sub-panel and have a 200A main breaker in the sub-panel.  The big breaker just serves as an obvious shutoff switch (and GFCI if it does that).