Getting a new animal into the house can be tricky. When I got my second kitten, my fist cat was over a year old, and she never lived with another animal of any kind. She's normally a bit unfriendly to new people, and she's really easily startled, so I was really worried how she's going to react to a very lively kitten. And, it was pretty much what I expected, there was hissing and arched backs when they first saw each other. So, I read all I could about it on internet, and started introducing them as slowly as possible. They were kept in separate rooms, and for a few weeks just sniffed each other through the door. It was very inconvenient for me, but I just couldn't risk it. In the meantime, I would carry toys and other object from one room to the other, since people suggest that mixing their smells helps. After that, I would place the kitten in the carrier, and let her in his room. I let her sniff the carrier, poke at it and sit on it until the kitten asked to get out. I did this for a week or two. Then I let them be together but only when supervised, they were still a bit rough with each other (especially the kitten, who was bigger then and was going through a biting phase). After a while, it was OK for me to leave them together. They don't cuddle together like I would want to, but they have no problems. So, I think that the best way to introduce them to each other is slowly, step by step. I can't really offer you any good advice, unfortunately, since your cats have already been living together for a while. One thing I've heard that helps when introducing a new cat, and might help in your case, is rubbing one cat with a towel, then rubbing the other cat with the same towel, then the first one again. That way you mix their smells together. If you haven't tried something like that before, it might be worth a shot.
