Someone asked me today about a flower essence that might help two cats get on: one is a real bully and the other just wants to be friends. You can energy test the flower essences that you have, but I came across this little blend that you may like to try. I haven't tried it myself because we have harmony in this household, but it reviews well. Click on the link below.
In addition, you can trace a heart (from a distance will work) over both their heart chakras to encourage them to let go of fear/anger/jealousy and begin to live from that loving heart energy of acceptance and safety.
Another way is to go backwards on the Triple Warmer meridian i.e. stroke along the side of the mouth (Mummy Licks) up over and down behind the ears and off the front paws at the same time - tittle tattle to him/her about being safe, loving, sharing etc.,