We've found ourselves in a seemingly impossible situation, so I'm turning to your collective wisdom in the hope that there is a solution.
We have two indoor cats, a nearly 16 year old girl (incredibly soft, has never so much as raised a paw to us) and a nearly 13 year old boy (very loving, but also unpredictable and prone to lashing out).
They have never gotten along. We introduced our boy when our girl's brother died aged 3 and there has been tension since day one. We've tried various things over the years, but our boy "polices" her, will jump on her randomly etc. They are both big cats, but luckily she is bigger and they rarely fight, it's just swipes etc. He has mellowed a lot but now she's older, it's clearly becoming more stressful. She keeps developing urinary tract infections, has lost a lot of weight, is eating less. The vet says it's stress, and we know that means it's him.
We've tried lots of things - feliway, anti-depressants for our boy. Separating them in the house is not an option, it would be too difficult to do. As horrible as it is to contemplate, it feels like letting our boy go might be the only option (other than putting our girl to sleep, which we would not do unless she was really suffering). Can anyone suggest anything else? We will all be heartbroken to lose him.