Why don't my cats get along?

Most likely the cat's feel threatned by eachother. Both cats want you, but they don't think that they can both have you so they're at war. There's not much you can do about it sadly, but I would recomened keeping them apart, because I had a friend where her two cats didn't get along and one hissed and gave the other cat a heart attack and now that cat is dead. =( For some particular reason your cats discided this is the cat they hate and they just didn't have a problem with the other ones because they didn't find them threatining. Sorry if this isn't much help.