Thank you for your question, my name isXXXXX and hopefully I can be of assistance.
If you haven't already tried pheromone therapy, that would be my first suggestion. There is a product on the market that is available through your veterinarian or online called Feliway. This is a plug-in or spray product that is a similar molecule to the "happy" pheromone that cats excrete from their salivary glands. Pheromones are unscented hormones that can trigger emotional and behavioral responses by acting directly on the signals in the brain. In this way, the "happy" pheromone can cause a sense of calm and tranquility for your cats, and may therefore curb or decrease the aggressive behavior between the two. I commonly use this type of pheromone therapy for behavioral disorders in cats with fair to good success.
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I wish you both the best and good luck.