It is not considered normal for a neutered male cat to act aggressively toward another neutered male cat. If your 9-month-old Siamese is biting or chasing your new Persian cat, several factors could be at play.
Possible Causes
Territorial aggression: Siamese cats can be especially territorial, and a new arrival may be perceived as a threat.
Dominance issues: Even after neutering, lingering dominance behaviors may surface, particularly if one cat views itself as higher in rank.
Misdirected aggression: Stress or anxiety caused by the new cat may be redirected toward the other feline.
Medical issues: Sudden behavioral changes can occasionally stem from health problems such as hyperthyroidism, so a veterinary check is wise.
How to Reduce Aggression
Separate the cats temporarily to let everyone calm down. Provide separate food, water, and litter boxes to prevent resource competition. Gradually reintroduce them through short, supervised sessions, starting with visual contact through a barrier. Reward positive interactions with treats and praise.
If aggression persists, consult a veterinarian or certified animal behaviorist for further guidance. With patience, most cats can learn to coexist peacefully.