If urinating outside of the litter box is a current thing, you should probably have that cat checked for a urinary tract problem. That is often the problem when the cat has been using the box. I think stress could cause it too, as Wayne mentioned. If it happened when one of them was a kitten, of course, you just need to use a cleaner with enzymes to remove the odor completely.
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A dog, I have always said, is prose; a cat is a poem. ~Jean Burden