It is tricky. Let me start by saying my first cat is a bitch. Befriending her is a hard thing for anyone to do, and it's more like a growling tolerance than an actual friendship. Cat number 2 is a cuddly teddy bear purr factory by comparison. She hated him at first. There was much growling from across the room or the other side of a door. Everyone's space is different, but to start out the new cat got his own space. We chose a space that was not very much part of the first cat's territory. He got his own bed, his own food and water dishes and his own litter box. They were not forced to share anything. Cat number one still got her food and litter and favorite bed in the usual places so if she wanted she could carry on, life as usual, ignoring the other cat. At first if they were in the same room she would stay in the highest place and growl at him, if cat number two made a move, cat number one would launch a full on attack. Honestly, I felt bad for the new little guy, he was a submissive kitten and got beat up. This went on for about a week. One day we introduced a new favorite toy, a brand new brown paper grocery bag. We put the new cat inside. Cat number one heard the rustling and came to investigate (she loves bags and boxes), for a long time she just watched him play with the bag, but eventually she couldn't contain herself. It was like a switch flipped and they played together. It was a bit rough at first. Over time they played more and more. Just this morning they were racing up and and down the stairs and swatting and wrestling without using claws or hurting each other. It took time. On occasion I find them in a bed together licking each others ears, but other days they are hissing and growling and won't get near each other. Cat number 2 eventually outgrew cat number one, number one gets frustrated that she can't beat him up like she used to and she doesn't automatically win, she is tougher by far, but cat number two outweighs her by 4-5 lbs. They will eat out of the same food bowl and drink the same water, but we still keep 2 litter boxes and they have their choice of spots to sleep so they can stay away from each other if they want. Every now and then they fight over a bed, and often things start as a game and they will play, but then one decides the game is over and will end the fun abruptly. I guess what I'm saying is that there is hope. They may not be BFFs but they can certainly share the space and will hopefully play and entertain each other as well. Good luck. It gets better but it takes time. It was probably close to a month before I felt they were friends, it was definitely a week or two before they really interacted at all, before that they kept to themselves. Honestly cat number two likes one of my dogs better and is constantly rubbing up on her face and trying to get in the dog bed with her

The dog never beat him up so I guess it makes sense that he likes the dog better.