When I brought my new kitten home one of my cats took an instant hate-on to him. She would hiss and growl and try to attack. He simply hid behind the couch, whenever he'd try to come out she'd attack. I wasn't sure how much he was eating, and he was just a kitten. This went on for about a week - long enough that I truly thought I was going to have to bring him back. It didn't seem like it would ever settle down - but it did. They peacefully co-existed for 7 years after that first week.
My advice to you would be to forget the "separate them" routine. They really need to work this out on their own. I think part of it is establishing the hierarchy.
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When I brought my new kitten home one of my cats took an instant hate-on to him. She would hiss and growl and try to attack. He simply hid behind the couch, whenever he'd try to come out she'd attack. I wasn't sure how much he was eating, and he was just a kitten. This went on for about a week - long enough that I truly thought I was going to have to bring him back. It didn't seem like it would ever settle down - but it did. They peacefully co-existed for 7 years after that first week.
My advice to you would be to forget the "separate them" routine. They really need to work this out on their own. I think part of it is establishing the hierarchy.