A rather expensive breaker in many cases.

And yes, a 2-pole GFCI breaker will provide protection for both 120 V legs and any 240 V loads (one of the reasons they are so expensive, and the relatively small volume used does not help).

I would run the MWC to the out building and then put a GFCI receptacle on each leg. If the heater or something on the other leg trips you would still have power and can reset right there. If you have a GFCI breaker you have to go to the house to reset and still wouldn't know which leg tripped.
3. sure. In fact a GFCI receptacle will accept two taps off the load side.

Be very sure if you go the two pole route to get one that handles LOAD NEUTRAL. You need to pass all the current carrying conductors (which on a MWBC includes the neutral) through the GFCI.

Thanks for the quick responses guys.

The 2 pole GFCI is expensive ($80+tax) and I would prefer using a GFCI outlet on either leg after I split the circuit but...

I can get away with only burying the UF cable 12" if it is 20 amp single circuit protected by GFCI.

I know that the MWC is recognized as a single circuit and had assumed that the GFI protection had to be before it split and before the UF cable entered the ground.

If that isn't correct, please let me know your thoughts. I would sure be much more convenient and cheaper to have GFCI outlets in the shop. Worse case, I know the 2 pole breaker will work now... Thanks again!

Somehow I recall that what you propose is not considered to be a branch circuit-- but cannot find the Article for the moment. Thinking that 30 amps is the minimum supply to a subpanel. But if it your proposal is properly considered to be a branch circuit, then Article 240.15 (B)(1) gives you a real break on the GFCI.

Thanks ronnatalie. The 2 pole GFCI breaker I have (Siemens QF220P) has three connections for the circuit wire, the center labeled load neutral. It also has a white pigtail attached to connect to neutral in the panel. Sound right?

That's the one you need, yes. There are a few that are strictly 240 out there (primarily for hot tubs) that don't have a neutral terminal nor the pig tail and hence won't work for your use.

I'm pretty sure that if you are going to direct bury at 12 you're stuck using the GFCI breaker. I can't recall any exception like bus is trying for. The MWBC counts as a single circuit which gets you out of some of the grounding requirements.

I thought I would post final results from all the questions:

Burial at 12" was OK, but must be residential application, rated UF cable and single 20 amp max circuit (MWC counts as single).

GFCI breaker was required (or outlet protection before it entered the ground if it wasn't MWC).

Thanks to everyone that offered advice. Ronnatalie your answers were spot on as were those that you provided to my earlier posts. I really appreciate your time and effort!

If you use rigid or intermediate metallic conduit you can get away with 6 inches of cover.

I do this regularly for post lights in front of houses.

You can take out plugs of sod and get the depth needed, and just place them back in the 'trench' and barely leave any landscape damage.