So I've had my current cat, Greta 3 years old female, for about 7 months now and she was originally part of a bonded pair but her sister died in the shelter. She gets really lonely and depressed by herself so I thought it would be a good and healthy idea to get another cat so she can have the social interactions she has been used to. My second cat, Keith 3 years old male, I've had for about 2 weeks now. He's a gentle boy who loves to play. I've been doing the room swapping, and mingling their scents, and having them try and get used to seeing each other but Greta is still very aggressive. She's growling, hissing, and batting at him through the barriers, and he has only retaliated once without hissing or growling of any kind. I have multicat Hormone diffusers and things that are supposed to lessen their stress a little bit. I'm really not sure where I'm going wrong or what else I can do to make this better. Should I just rip the band-aid off and let them mingle?