Possible to ground-fault protect both legs of a MWBC with two single-pole breakers?
Never gonna work. Sorry, but even if you might be able to get one at a time to work, GFCI on MWBC is based on the breaker comparing "all hot" to "all neutral". Two electrically separate breakers simply can't do that. The handle tie requirement satisfies the general MWBC safety issue of making sure that if one is off for maintenance then the other is off as well. Options:


Double GFCI breaker
Two single regular breakers with handle tie and install single GFCI where needed at point of use (e.g., kitchen or bathroom).


GFCI is perfectly effective at point of use. AFCI is best at breaker or very close to it.