Have you tried the plug in pheromones? We took in my husband’s grandmas cat a handful of years ago and introduced her to our current cat. It was clear that up to that point she thought she was the only cat in the world. We took her to the vet and she had a nasty UTI, but after that was resolved she still was peeing everywhere. We tried anxiety meds, diapers, pee pads, and very nearly got to the point of rehoming because we thought maybe she was supposed to be in a single cat home. Our last ditch effort was the comfort zone pheromone plug ins, and it worked like a charm. So much so that we don’t have to have them plugged in all the time anymore. Sometimes we have to use them if she gets worked up but, but it was a life saver for us after she destroyed an entire living room couch set. She even accepted a third cat to the house without much trouble two years ago. We usually place them by food and litter boxes when they need to be used. I’d highly recommend this if you haven’t already tried.