I have tried this protocol and recommended to adopters at my previous job as well. While the cats are separated, I would also add to switch out their bedding. Take whatever the cats are sleeping on and trade it. I have a super cranky cat who does not like anyone, including her son cat who she has lived with for the past 14 years. When my cat was about 7, we had a new roommate move in with her cat. I totally thought it was going to be a disaster, but we went very slowly...very slowly....very, very slowly. We put the new cat Zoe in my roommate's room while Mama had run of the rest of the house. We switched out their bedding and feed them on their respective sides of the doors. Mama isn't huge on food, so we also put catnip (which she loves) outside of Zoe's door. The key is to find whatever the cats like to create a positive association with what is on the other side of the door--food, treats, toys, you, a special bed, Feliway, etc. After about 3 weeks, we let them come out and sniff each other. Of course, Mama swiped and hissed at Zoe and Zoe hissed back and ran away. We separated them again and tried later that day. After they were calm around each other, we let them have supervised visits ending in catnip. After a week of supervised visits, we let them be together unsupervised. They co-existed but were never friends. We lived with Zoe and her mom for about 2 years without any major incidents.
#AnimalBehavior,TrainingandEnrichment