Multiple mods won't append or merge, there would need to be an interface for conflict resolution - which would be more trouble to the end user (and modders) than it's worth. Though to be fair, NWN actually had a working CR model that in my opinion went a long way to keeping the game alive well after its shelf life had expired.
If you're doing things the "manual" way like some of us are right now, you'd have to hand edit the TXT file to have both sets of changes. :\
I'm thinking that I could side-step the fish issue (the "fissue"!) by adding a whole scad of recipes in ItemCombos for every kind of fish, as opposed to fixing the FoodFish category itself. Dinner (elven stew) is pretty darn useful and fish seem to be much easier to come by than raw meat, so it would be worthwhile just for my own playthrough, hopefully for others as well.