My parents cats keep peeing in laundry piles

My parents have two male cats that are about 10 years old. They grow up together their whole life and generally get along. They share one litter box (the one you rotate and all the dropping is in a drawer) that gets cleaned once a day. The problem with the cats is the minute a pile of laundry is left unattended they immediately pee on it. What advice do you have to train them not to do that?