In my opinion these fish should not be separated. It does no harm to leave them as they are. That top fish seems very happy swimming around chasing feeders constantly. The bottom fish also seems to be doing OK as it is eating. Isn't one of the signs of a stressed fish is appetite goes down? I believe neither of these fish are stressed, this in no way can be considered animal cruelty as you are in no way being cruel to the animal by letting it live. If they are meant to die they would have died a long time ago.
