We introduced a second cat last year to our 6 year old who we feared was a bit bored and lonley.
I was concerned because she is very set in her ways and has been used to being a 'loner', so I did some research online to find the best way of handling the situation.
The trick is to keep both cats apart for a couple of weeks and slowly introduce them to each other during that period. Keep the new cat upstairs or in a bedroom and then the existing cat have free reign over the rest of your home. That way they won't feel like their territory is threatend. The cats will be aware of the new addition and the idea is to let them sniff each other out through the door and get curious. Eventually, you'll supervise opening the door slightly so they can see paws, tails etc and keep going until you can place something in the door way so they can both see each other fully. the final step is to allow the old cat to enter the new cats room and let them play and get on with it. Within a few more days, depending on how that went, you can let them out together.
It's important to spend a good amount of time with both cats, especially the existing one and shower them with love. If your cats have blankets, then after a week switch them over without washing them. That way they get used to the scent of each other.
Now, having said all this, my other half couldn't wait to mix them up and let both cats out together in the first night. Such a child!! We did make the mistake of keeping our old cat upstairs and not down, which made her feel 'left out'.
She was very unsettled with the new addition and made some very odd noises - not growling but a clicking type sound. We couldn't leave them alone as she'd swipe for the other but eventually we got there.
I can't say that they are best friends but they do play together and have been spotted napping too. sometimes it gets a bit aggressive and they both are happy to inform the other when a lines been crossed!
It does work and I know they are both happier for having each other around.
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