Follow our “golden rules” on resources.
Duplicate: each cat must have their own food bowl, water bowl, litter box and scratching post
Separate: Spread your resources around the house. Ensure each cat has it’s own SEPARATE “dining room” and “bathroom”! This provides each cat with some privacy. Use the whole room. You can put some bowls higher up and some lower down to provide space.
Conflict management, DO NOT punish your cats when they show signs of conflict. This would cause stress and therefore lead to more conflict. Provide more than one method of escape (exits) in each room. Also can provide covered walkways or tunnels so cat’s can get around and avoid conflict. Provide plenty of places to hide/rest and perch. If conflicts cause signs like urine spraying or scratching.
If your home has frequent house fights, it’s important to do your best to stop it. For your cat’s health as well as you.
This is a process that may require patience and work at training them as it can take a lot of time. But stay with it as it may get you your desired outcome, however also be aware that some cats are completely solitary and may never get along.
Cats are naturally solitary animals and can be very territorial. The fighting may occur due to territory. 
Observe the cats carefully to see if you can pinpoint when and where the unwanted behavior happens. For example, does it happen around mealtimes or when you are giving attention to one of the cats? 
You can help by adding more territorial space and spaces for your cats to go out their day, sleep and pass one another peacefully or escape one another. 
This can prevent the cats from having to share climbing, hiding, and perching areas where fights can break out. And prevent stress or avoidance from fear of conflict. 
You should increase resources, your cats should have minimum and separated.. food and water bowl each and one litter box each if not more, scratching post each and toys or cat trees/perching areas. The more the better and in different locations.
If one cat is the aggressor try and intercept things before they occur or get bad. Redirect their behavior with an interactive toy/string and lure it into play. 
You can also interrupt with a sharp and short sound or ‘no’ and once they are away and cam, then you can reward this good non-violent calm behavior with fuss, food or a fun toy.
But avoid rewarding behavior you don’t want to see. Try not to give food or toys after the fight has broken out. 
Seeking help from an animal behaviorist may be very helpful to work out what to do, how to manage and control this situation. 
A visit to the vets may be necessary. Some health issues can make cats angrier than usual as a symptom. Especially if they have only just recently started not getting along when they used to, or if your cat has now become more aggressive. 
Treat the cats as though introducing them for the first time. GIve them their own areas and then make slow and short introductions. 
Use controlled situations to expose the cats to each other. Make sure the cats always have an exit to make them feel comfortable and allow them to leave if things get too much. 
Try to slowly see one another and then progress to being in same room whilst both being fed cats tasty foods or try to get them to engage in play so they associate being near one another to fun and positivity.
Never force them into or to leave a situation.
Give each cat one-on-one attention. If necessary, put one cat in another room while with the other cat. During one-on-one time, hand-feed treats or initiate playtime.