Good Morning and welcome. I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
I am so sorry to hear about your situation...I am very sorry that it has taken us so long to answer your question.
From what you have told me, it appears that Cooper is suffering from some type of stress that has caused this inappropriate elimination. This could be due to a territorial issue or some other stress. Most likely this is a terrotory issue with your other cat. The first thing to do is to have your vet examine Cooper to be sure there is not a medical reason for this behavior. Since you have already tried the Feliway, then other things will have ot be done. Next, make sure to change all litter boxes to different places especially where there is little traffic. Keep these boxes extremely clean.Have at least 2-3 per cat.
Try using different types of litter and make sure to clean these areas with an odor eliminator such as Nature’s Miracle.
Use aluminum foil or double sided tape in the problem areas which may discourage using these spots.
If this is not sufficient, your vet has medication that can help with this problem and, if used along with the Feliway, hopefully, will be successful. These medications are antianxiety drugs such as Fluoxetine or Clomipramine. Here is a website that has more tips:
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Does that make sense to you?
Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please Reply so I can clarify for you
Sincerely, Dr. Loretta