To get a MWBC working, with both legs GF protected, I installed 2 Single Pole GFI breakers, on adjacent slots, with a handle-tie. Since there are 2 lugs for Neutral wires, and only one neutral coming in, I used two pieces of white 12 awg to wire nut to the neutral coming in.
It doesn't work, trips as soon as it's flipped on.
I even opened the circuit at the first box, (so i kknow it's not an actual ground-fault) and it still trips, even though neither leg of the circuit could possible be conducting at all!
I was told I could use 2 single-pole breakers w/ handle tie to get both legs of the circuit GF protected, but this appears to not be the case. Am I wiring it wrong? Also, how is it possible for them to both trip when neither is conducting?
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.
Whoever told you that was referring to ***A***FCI breakers, and only in the context of a GE panel.
In every other case, they don't know what they're talking about.
A GFCI operates by comparing the currents on all the normal/intended conductors to assure that
current in = current out To do that, it needs access to all the conductors at once (not ground). That way it can account for the several ways current can flow in in MWBC.
Theres another mistake here: paralleling. You are not allowed to have two redundant routes for the same current, and you created a situation where neutral can go through either GFCI. That is the thing never to do.
Paralleling is allowed with large cables and special equipment made for the purpose, but "#12" and "two GFCIs" are definitely not it :)
If the "someone" happened to be a person at a big-box home improvement store, that is a sign to stop listening to those people. Their advice is no good. A manager at a real electrical supply told me he visits big-box stores regularly to size up the staff and hire away anyone competent. That is the place to go for electrical gear. Tell them you are sick of being overcharged by Home Depot, and see what they say.