Cats can get along and should. It may take a little coxing, but it can be done. Here’s how:
Separate the cats, if need be, and gradually reintroduce them over a period of a few days or weeks time. Place the more aggressive cat in her/his own room with food, water, litter box, bed and toys. Feed the cats on both of a closed door, so that they become familiar and accepting to each other's scents with something pleasant such as good quality cat food. After a few days, permit them to sniff and look at each other through the cracked open door. If that goes over well, the next day try letting the aggressive cat out on a supervised visit, then continue with the following steps. Go slow and easy, let the cats choose the speed at which they are reintroduced and never force the cats to interact with each other. Separating and reintroducing the cats is typically only necessary if the fighting is regular or if they are fighting hard enough to cause injury.
Employ the felines in play-therapy to burn off hostile energy and improve the cats' self esteem. Playing with the aggressive cat will teach her/him to direct her/his vigor toward a healthy object, such as a catnip mouse or rubber ball, rather than his/her kitty companion.
Since cats want to feel like triumphant hunters, playing with the picked-on cat will boost her/his confidence and help her/him stand her/his ground if the other cat becomes intimidating.
Play with the cats separately, in the beginning, behind a closed door if you must, and bit by bit play with them in nearby rooms, then in the same room, then play with them together. A couple of 15-minute play sessions per cat every day are a great start, and you can increase that if your cats like it. Praise them at the end of each play session, and give them a treat or so that they feel like they earned something.
Give the cats a reason to like each other by giving them a treat and praising them whenever they are peacefully in the same room together. Use the treat to lure the cats closer to each other, but stop if the cats seem nervous or hesitant.
Sidetrack the cats if you think one is about to attack the other. Idyllically, try to engage the aggressive feline in play to direct her/his energy somewhere else. If that won't work, close a door or else try to separate the cats before they can begin fighting. As a last alternative, make a loud noise, such as clapping your hands or shaking a tin can filled with pennies to startle the cats and break up a fight.
Pay lots of attention to both felines, when you are alone with one or when they are in the same room together. Contented cats that feel loved are less likely to fight.
You should comfort the picked-on cat after an attack, but most expectedly he/she's going to be too afraid to want any attention. Play with him/her or pet her if he/she'll permit it, or try just sitting in the same room together. Let him/her hide under the bed for a while if that's what he/she so chooses.
Make certain there are enough water and fool bowls, litter boxes, toys and cat trees for every cat so that they don't have to go halves with if they don't want to.