Sorry, but we do not offer any GFCI devices that are suitable for split feed wiring. If you want to use one hot and its accompanying neutral, you would be able to use the GFCI. Make sure the other circuit is still intact though to maintain continuity, of course. This should have no bearing on the "unused" circuit.
I don't know if you are still watching this, or if you have found a solution. I have the same problem as you do. At least I think it is the same or very similar. I have four duplex outlets in the kitchen. There are two circuits for the kitchen All of the top outlets are on one circuit and all the bottom ones are on the other circuit. In my case, I can see how the power gets to the outlets. They are all on the same run of 3-wire (plus ground) Romex. The red wire goes to the top of all outlets, The black goes to all the bottom outlets and the white (neutral) goes to the other side of all outlets - both top and bottom. (Red is hot for one circuit and Black is hot for the other circuit.)
You probably will need to put a combination circuit breaker/GFCI combination to handle this situation. It would be a 240 volt double breaker with GFCI (the breaker would feed two 120 volt circuits, just like you have). You would need one that would fit your breaker box. For instance, my breaker box is made by GE, and I am looking at the GE THQL2120GFP combination circuit breaker/GFCI. You can find it on Amazon, and probably on the ge.com website. Even if you don't have a GE circuit breaker box, it would give you an idea of what to look for to fit your breaker box. You could keep your existing outlets.
GFCIs work by detecting the current coming in on the hot side and comparing it to the current going our on the neutral wire. If they are different, there is a "ground fault", perhaps thru your body. Things get more complicated when the circuits share a neutral - which is why it has to be checked in the double breaker where everything ends up. I won't go into the details here, but it has to monitor both hot wires and the single neutral both circuits share.You might also want to see www.afcisafety.org/qa.html for a discussion of split wired receptacles.
I think that will work, but I haven't tried yet.
I need to install GFCI plugins on either side of my kitchen sink (older home). The problem I have run into is that each of the current receptacles is wired with two hot lines in such that the upper receptacle is on one circuit and the lower one on another. I suspect this was done at some point to allow a greater degree of load balancing on a limited number of circuits. There is no "continuation" load line going away from the receptacle, so I suspect these receptacles are pig-tailed off the main lines somewhere in the wall.
I suspect there is not a GFCI made to directly address this situation (If there is, please let me know which one.). Assuming there is not, I was wondering if I could just terminate one of the hot lines coming in and wire the GFCI with only a hot and neutral line along with a ground?
I would also like to know what, if any, issues this may cause with regard to other wiring beyond the GFCI. I imagine none since there would be no load out as is currently the case.