I had two older cats who were a bonded pair. After one died, I rescued a young adult. Not sure of exact ages, but safe to say resident cat is a senior though fit and healthy, and new one is a young adult. Maybe I should have gone for another senior, but this one needed a home, and its done now. I did a gradual intro with scent swapping. There was some hissing and growling the first time they saw each other. Now they're...okay. They can be in the same room and will sleep in each other's presence, as long as the younger one doesn't get rambuntious/in the older cat's face. If she does, the older cat hisses and swats. Youngrer cat doesn't retaliate. This makes me a bit sad because older cat was never a hissy cat before.
They can both live here - I have space, and they can have seperate exits and entrances. If I feed them treats close to each other, they both eat the treats and don't try and fight each other or anything. I don't need to leave them alone together, there's space for them to be seperate while I'm out and for the younger one to have her one entrance/exit. But it seems a bit - sad? Am I projecting too much human emotion here? Is there any hope they can actually be friends? Its been a few weeks now and they seem to be stuck at resident cat's attitude being "fine she can live here, just keep her out of my face". She's still eating, sleeping, and toileting normally so she's not unduly stressed or anything and I make sure she has her protected alone time/sleep time, which she likes and needs.