Cilliliz, do you think that if you just let the two out loose around each other all the time they will eventually just get used to each other? I am not sure what Callie is doing exactly...is spraying involved? That might be a rather difficult thing to overcome....but if they just hiss at each other and maybe claw at each other, I would imagine that they would stop at some point.
With dog Bosco and Freddy I just left them to roam free in the house 24/7 and after some weeks Freddy finally stopped hissing at the dog...now he follows Bosco around like a little pal.
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It isn't just the simple hissing behavior. I've been through that with other cats in the past and that's no big deal. This is much different. Callie literally stalks her down and pounces as if she's prey, and it isn't a game. I've never seen anything like it. She isn't spraying - she's just leaving her calling card in the middle of the hallway that leads to her litterbox. Right where I can step in it, I may add.
I had to take Boo to the vet tonight for her last round of shots. They weren't surprised with the problems I'm having and said this isn't uncommon with calicos. One vet suggested keeping them totally separated for a month, when I said it's been a month he said "or two". They didn't think I would have much luck in the long run. So, one of the vets said she may know of a wonderful cat lover who's cat has very serious cancer. She doesn't know if she'll be looking for another cat, but thought of me right away and will call if she knows anyone who would meet my approval.
I have my former neighbor asking around her friends (mostly seniors). So, if it isn't a young girl, it may be a senior living alone. I am not going to take her to the shelter. They recently had a sign on the door saying they had too many cats and any cats dropped off would be euthenized. (or however you spell it). I refuse to take that chance. I'll keep her until I find a home.