You can see if cat appeasing pheromone in Feliway will help reduce the tensions between the two cats. You can read about this here
http://www.catfaeries.com/feliwayfaq.html
and its found in pet supply stores or online. While its main function was for use in cats who were spraying its also been found to do a lot for aggression issues.
You may also want to ask your vet for a referral to a veterinary behaviorist. Sometimes medication can be used to change behavior and once the new routines are established through behavior modifications the medication is weaned away.
Hope this helps you!
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