Kitkat vs Fon (still at it)

Two months in and the house is still a war zone every evening. Fon is the young one, 1.8, and she thinks everything is a game. Kitkat is 8 and wants her space, so when Fon jumps on the back of the sofa Kitkat just hisses and slaps her. Then Fon screams like we are killing her. Very dramatic. I read online that we should use the same food bowls and the same litter tray so they smell the same. Sort of works. We also do the slow intro with the door, two bowls on each side and a towel with the other cat's smell. Kitkat did not care, she only eat when Fon is far away. Last Tuesday they actually slept on the same blanket for 10 minutes before the fight started again. I was so happy I take a photo, then got scratched on the wrist when I moved closer. Worth it. My husband says just leave them alone, they will figure it out. I think he is right but I cannot sleep when they are in the same room and one of them is growling. Maybe I should buy a second cat tree so they each have a high place. Any idea if that helps?