I have a 15 amp breaker to my pool light and a 20 amp 2 pole breaker to the pool pump both using a common #12 neutral. How do I wire GFCI breakers to replace existing breakers? Wires are underground and can not be changed/added. #12 wires are aluminum.
I find it highly improbable that these circuits were wired with /4 cable back in the age when people were using aluminum. So I will proceed assuming that there is a 12AL/2 cable for the 240V-only pool pump, and a 12AL/2 cable for the pool light/recep.
12 AWG Aluminum wire requires a 15A breaker. Using a 20A breaker is out of the question. This will be ample for lights/recep and a 1-horse 240V motor.
I would use a common, 120V, dual-mode AFCI+GFCI breaker on the 12AL/2 cable for the pool light/recep. That's done and dusted; any GFCI receps on that circuit which are outside can be replaced with plain outlets and used indoors etc. In fact they should be replaced with plain CO-ALR outlets; because most GFCI outlets do not support aluminum wire.
The reason to use AFCI is the aluminum wire. Small gauge aluminum has a history of problems, but 90% of that is because of using inappropriate receptacles which are not rated for aluminum (or, hastily rated for aluminum with inadequate testing, e.g. "AL-CU" per the 1970s; the R in CO-ALR stands for "revised"). The AFCI takes care of that.
A 240V, 2-pole GFCI breaker (about $80) goes on the other cable to the pool pump. The breaker pigtail goes to the neutral bus, but the neutral terminal on the breaker is unused.
All aluminum connections should either land on CO-ALR receptacles or switches; or on Alumiconn (mini lug connectors). If able, a CO-ALR recep makes two aluminum-copper splices and it's cheaper than an Alumiconn. I'm not sure they make 220V receptacles in CO-ALR.
If the pool pump for any reason has tapped the neutral of the other circuit, you will need to remove that. A normal pool pump does not need it. If yours is a 120V pool pump, you'll need to rewire that other circuit as 120V instead of 220V, but I doubt that.
Two different circuits from two different GFCI breakers should NEVER share the same neutral wire.
You will be unable to install and have work a GFCI with this non-standard and unsafe configuration.
If you fix the wiring, to meet code, you can install a GFCI on the 220 V circuit and a separate on one the 110 V circuit.
GFCIs work by looking at the sent current, and the return current through the neutral. So they need their own neutral for each GFCI. A 220V GFCI is a little more complex in that it matches current though L1 and L2, as well a neutral. However a neutral is still required.
The business about sharing a neutral wire among those circuits is sketchy. It's possible that the pump runs on 240 volts and so doesn't actually use the neutral at all, though. As others have pointed out, there are likely some code violations embedded in the arrangement.
It sounds likely that replacing those two breakers with GFCIs could be unsuccessful because of the shared neutral. There is another approach that would work, however. Understand, though, that this does not eliminate or mitigate the problems of a potential shared neutral -- it merely re-arranges the situation so that a GFCI can function.
You could move these two circuits to a separate sub-panel. Use normal breakers in the sub-panel -- even literally move the existing breakers, if you choose a sub-panel in which they're compatible. Feed the sub-panel with a two-pole GFCI breaker. The GFCI protection will extend to all circuits originating in the sub-panel.