In August, I adopted a one year old female cat. My first cat is a three year old female, who I’ve had for over a year. I’ve been going slow with their introduction, but we keep getting stuck at the step where I introduce them face to face. I have a baby gate up in the new cat’s room (my office) and whenever she gets close to the gate, my older cat will hit her through the gate. At night, I let the new cat roam the house (since she was getting antsy and always trying to escape her room to look at the rest of the house), and I keep my first cat in my room. Whenever the new cat comes near the door, my resident cat dives under the door to swat at her. Whenever I let them close to each other, my first cat will almost immediately start swiping at the new cat, who just hisses and presses her ears back. She never retaliates.
I’ve been trying give them positive associations with each other by giving them treats on either side of a door with a small crack, which has been making my new cat more open to my older cat. They’ll eat peacefully until the treats run out and then my older cat will start swiping again.
I believe it’s territorial aggression. Whenever my older cat has some sort of encounter with my new cat, she’ll start viciously rubbing her scent on everything near her. I’ve tried the Feliway near the door to the new cats room, but it didn’t seem to have an effect and my older cat would continue to swipe at my new cat. I’m not sure what to do to get my older cat to tolerate the new cat.
My older cat hasn’t yowled or hissed at the scent of the new cat since the first week she was here, she doesn’t get upset being in the new cat’s room (just starts rubbing herself on everything), but just can’t seem to stand the sight of her. There hasn’t been any blood drawn or injuries because my resident cat doesn’t seem to be using her claws, and my new cat is warming up to the older cat because she tries to seek her out during her active hours, but my older cat is just not accepting her. What should I do?
Edit: Both of them are spayed.
Edit (01/09/21): I've been draping a blanket over the gate so the girls can't see each other during the day and have been taking constant breaks out of my work day to spend time with my resident cat, so she won't get jealous. At night, my resident cat seems less aggressive with how she swipes under the door at the new cat. They also seem to be playing with each others paws under the door. In reply to the comment suggesting to go back to the step where I'm swapping their scents, they still don't react to each other scents, both on a cloth and around the house; they sorta just sniff whatever it is and keep moving. I've started giving them treats on either side of my bedroom door in the morning with just a crack in the door. I know this is going to take time, but am I heading in the right direction?