Are they male cats? My room mate had this problem once and it was terrible. The reason is this is occuring is that, once the cat sprays or unrinates in one spot the scent makes them remark their "territory" in the area over and over again. And the cats could be actually "battling" for territory by spraying or urinating where the other cat has or marking his own territory.

 ARE YOUR CATS SPAYED OR NEUTERED IF NOT IT MAY BE THAT THE FEMALE IS IN HEAT AND THE MALES RESPOND BY SPRAYING. IF YOU HAVE THEM FIXED IT WILL HELP