Tricky problem, and happens more than many folk would think, but cats aren't dogs and won't necessarily think "whoopee, another cat to play with". The comments from Calipso and VioletSky are sensible and accurate. There was a show called (?) "My Cat From Hell", (possibly still on Animal Planet, or similar channel?) with an American cat behaviourist, dealing with a whole variety of cat issues. Many cats in the States live as indoor cats, and the "warring cats" syndrome was one he dealt with occasionally. It's ages since I saw that show, but the solution (to try and keep both cats) requires some work, which not every human will do, but it's worth a try. One method involves mealtimes, making sure that #1 cat gets fed first (even by just a few seconds, it's a symbolic thing) and also that they can eat quite close together, though if necessary separated by a barrier, but so they can still see each other. Other factors involve where they like to hang out, so each has their own space (this is complicated by some cats shifting their preference every week or two!). This type of info and more should be available somewhere on the i'net, you've just got to find the right phrase to Google (warring cats?).