It does take time... I had two cats who were sisters. One died about 2 years ago. They were both very bossy cats but could hold their own against each other. It made it challenging to have any other besides them. After the one died, I picked up another cat from my mom who seems to collect them. I kept them completely separate (they could smell and paw fight under the door) for the first month or two. Gradually I was able to let them interact for short periods of time. It seriously took well over 6 months for me to be able to leave them alone together, and over a year for the fur fights to stop. Now two years later, I have a picture I took that completely shocked me - them both eating from the food bowl at the same time!

I didn't find the feliway to help much. I had slightly better luck with Bach's Rescue Remedy. Either dropped onto their gums or in their water.