Corrine K. has a pair of cats named Thelma and Louise. T&L have some issues with getting along. Corrine writes:
Corrine, the first step is to determine whether or not Thelma and Louise are actually fighting or just playing rough. This can often be difficult for humans, but a good indicator is the amount of noise the cats make during the interaction. If it's rough play, it should come to an end when one cat makes hissing or yowling sounds. That's her way of saying that she's had enough. If the "play" continues after that, it's more likely to become a problem.
It's up to you to determine whether your cats are playing or fighting by observing their behavior and thinking like a cat. Some cats don't learn boundaries when they're kittens. This can happen if they're taken from their mother too soon or if the mother was socially challenged as well. These cats often bite inappropriately and fail to get the hint when their rough play isn't welcomed. In these cases, you can act as the mother and help your cat learn boundaries. Usually, a gentle intervention is all it takes. Separate the cats and leave it at that unless the row starts up again, but always do so with kindness and care. Never punish a cat by yelling or striking her. If you do so, all she will learn is that you're the one to be avoided.
If you determine that your cats are truly having a problem getting along together, it's time to examine the causes. Most involve a territorial dispute of some sort. Cats evolved to be solitary hunters who share resources by cleverly avoiding direct competition. Two cats plus limited resources in a closed environment equals potential conflict. And by "resources", we don't just mean food. Warm laps, window perches, cuddly beds, and scratching posts can all be resources to cats.
Your first step should be to provide separate eating areas that are out of view of one another. Each cat should feel safely isolated from the other during meal times. The same goes for litter boxes. You should have three litter boxes (one more than the number of cats) and these should not be visible from one another. Yes, in many apartments this will be difficult, but do the best you can.
Your cats also need plenty of places to relax, including vertical climbing space. The addition of a couple of tall cat trees will certainly help. Each cat needs to feel that she has the resources she needs when she needs them. If they feel as though they're competing with each other, the result will be conflicts. This extends to lap time as well. If both cats want to be close by at the same time, a pillow on each side of you will have to do. Share affection equally and make it clear that you love both of them. The result will be a more civil household.