If the accidents started since the cats stopped getting along, then this might be a case of territory marking. Key to resolving the problem is getting rid of any existing odor (clean the patch daily for at least 2 - 3 weeks after the last accident) and decrease stress between the cats. Do this by providing high perches, hiding places, separating their food bowls so they each have their own food in separate locations, and providing 2 routes through each room so they can avoid each other. Do not tell the cats off, as this increases their stress.