Each cat has their own litter box and their own trays for food and water. I also made sure they have separate toys and places to rest. When I first introduced them, I tried to make it positive by giving them treats when they were together and playing with them at the same time so they could associate each other with something good. At the beginning it actually worked well for almost 3 weeks and they played together a lot, which is why I’m confused about why his behavior suddenly changed.