Aggression between household cats
Mini came to live with Mrs. Hardy and Benny about a year ago and the aggression between the two cats began immediately. Cats are territorial and once fighting has ensued, they will react quickly and remain reactive for some time. The two cats need to be re-introduced and taught to relax in each others presence. It is usually helpful to re-direct the aggression to some extent.
Separating the cats
For the first three or four days, the cats are to be separated into boring environments anytime they cannot be supervised. No toys, no food and no owner in this environment. Just a litter tray, water and a soft place to sleep. The cats are then brought together for short periods (5 or 10 minutes) several times a day. Distraction (see below) can help stop acts of aggression. Try to anticipate any aggressive acts. Stalking, glaring, hissing should all be broken off by the owner with the distraction.
When they are brought together
One of the main things to address is the enrichment of the cats environment in order to give them more things to do. Try creating high places in which the cats can climb or jump. Use your imagination! Try to link all the good things i.e. foraging, toys, and your attention to the cats being together.
Re-directing the aggression
Try to find stimulating toys or hanging/swinging objects which will entice the cats to attack and play with them. Hopefully, the cats will learn to look for their toy when the need to behave aggressively.
Teaching them to relax
Also, try to teach both of the cats to relax after a hard play with the toy or even when they are all together with you. Try to put the cats in a physical state of calm when they are around each other.