I have two spayed females that have been together for 5 years. At the beginning of July, one started attacking (not play fighting) the other. They've been separated since then. The aggressor has been in one room and the other is loose in the house. I've gotten them to the point where we can have a pet gate up so they can see each other. At times we can get both cats close to the gate with no problem and other times there's hissing. In reality, this situation is way better than when this first started. My husband and I will sit on either side of the gate and play with the cats, pet them and give them treats so they start associating fun time with each other. It's slooooowly getting better, but any other hints you may have would be appreciated. We have Feliway diffusers going in the rooms where the cats hang out the most.