Slowly introduce cats over days to weeks so they can become comfortable with each other's scents before meeting.
Slow Introductions
Separate fighting cats immediately into their own "safe" rooms until they calm down.
Separate Them
Distract fighting cats with toys that spark playful "hunting" energy instead of aggression.
Redirect Energy
Try synthetic feline pheromones like Feliway to help calm tensions between feuding cats.
Use Pheromones
Provide multiple litter boxes, food/water bowls, perches, etc so cats don't have to compete.
Adequate Resources
Use treats and toys to redirect inappropriate chasing and pouncing between cats.
Discourage Chasing
Ask your vet for additional advice if cats remain hostile, stop using shared litter box, hide frequently or seem stressed.
Consult Your Vet