This is totally up to you but you need to make sure that you feel okay with whatever you decision is, and be ready to explain this to the previous owners if you may happen upon them again.
Honestly, if it were me, I'd keep all the leghorns. I have an issue with not giving away chickens, honestly. The first time I ever hatched some I told my friend that if more than three hatched, I'd give her the remainder because I didn't want that many chickens. What happened? Six hatched and I kept them all. A broody hen started sitting on eggs immediately afterwards: Five of hers hatched-- I kept them all! So maybe I'm not the best person to give advice about this.
Anyway though, in all seriousness, there's no point in keeping chickens you don't want to deal with that you don't need for eggs. Someone else out there would probably gladly take the leghorns and give them a good home. If you really don't want them and they won't be useful to you, I don't see the harm in selling them as long as you try to find them a really great owner. If the previous owners do happen to figure out about it some day, you could just assure them that you were very selective and careful about where the leghorns were rehomed and that you are confident that they are living the best lives possible.
If I did give away chickens to somebody and they told me, "I can't keep all of them because I wanted a different variety of chicken, but I did give two of them away together to someone who I know is taking good care of them." I wouldn't be upset at all. I'd be happy that you were honest with me, that two went away together (so I'd know they had other chickens to live with), and that they were living somewhere where they were wanted. I'd also be happy that you got the chickens you really wanted. If the previous owners want, and the leghorns' new owners are okay with it, you could even exchange their names with each other so the old owners can see for themselves that the chickens are taken care of. I don't think that would be immoral