Hey now don't be a bully. I wouldn't have asked if I knew!
I'm guessing you need to give them to individual moogles as with the letters, but that's just a guess.
I know you'd rather hold two Kupo nuts at once but, unfortunately, if I remember correctly, you have to return them one at a time to the first Moogle that asked for them and he'll give you an item. Most are average but sometimes you get something nifty, based on something something. I don't remember how that worked.