I adopted a new male adult to join my male adult and the new one wanted to be friends. The established one wouldn’t accept him. They didn’t get along until we moved and neither had a territorial claim.
I think you can take objects (like a blanket or towel) that they have used and get each other used to the scents. I have only tried this once (only adopted a new kitty with an established kitty twice). Not sure how effective it is.
Most important, to me: pet the established kitty first every time you come into the room. A kitty died and I brought in a new kitten. I always talked to and pet the adult first so that she didn’t feel like she was being replaced / reduced jealousy. I don’t know how much this helped, but I think it was better than had I pampered the kitten first. That’s just vibes, but it sure didn’t hurt.