GFCI with 2x 20 amp 1 pole breakers
You have the wrong breaker for what you want to do here

The Eaton DNPL (aka BRD) type of breaker is a tandem or double-stuff breaker, that has two circuit breaker mechanisms inside it that share a line terminal, but have separate load lugs.  These are used extensively to "cram" circuits into a panel, as we see in your panel.

However, since both of the breakers in a double-stuff are connected to the same leg or phase of the service, you cannot use one to feed a multi-wire branch circuit (like the one you propose) without overloading the neutral, since the hot wires in a MWBC must be on different legs or phases to allow only the difference in loads to flow down the neutral, rather than the sum of the loads.

If this were a regular branch circuit, you would need to replace the bottom rightmost DNPL1515 and the empty slot with a DNPL152015, then land the wires from the old DNPL1515 on the two outer poles and the new 12AWG wires on the two inner poles.  However, this creates other problems, so read on for details.

You don't have the space for a GFCI breaker

The bigger problem for what you want to do though is that you need a two pole GFCI to protect a multi-wire branch circuit, and those only come in breaker form.  However, there is no such thing as a double-stuff GFCI breaker to begin with, as the electronics required to make a GFCI work take up too much space to be fit into a single breaker package along with the two breaker mechanisms.

So, you'll need to put the GFCIs somewhere else -- this could be a "spa panel" placed inline with the circuit with a 2-pole GFCI breaker in it, or a pair of GFCI receptacles or deadfronts at the "end of the line" for the MWBC in that you would have to have the two "sides" of the circuit have separate neutrals from that point on to avoid confusing the GFCIs with improperly divided neutral currents.  In this situation, you'll still need to use the DNPL152015 quadplex breaker in the main panel, as described above.