It's so simple... CLEAN THEM!
Out of sight, out of mind? No excuses. The cats are still going to the bathroom at least a couple of times a day. Imagine being a creature whose nature is to be clean. Then picture that box of grit filled with pooh and pee, take your shoes off and step on in. Delightful, eh? Just Do It! If your boxes are in the basement or hidden away, they still need to be cleaned Every Single Day.
One Cat = Two Litter Boxes. Yes, you should have at least one box per cat. And for one kitty, two boxes would be welcome. I have a cat who pees in one and poops in the other, exclusively. When I say "litter box" I mean a BIG litter box. A nice big box with lots of litter in it. Not an inch or two. You may think this is costly, but if you are cleaning it daily it lasts a really long time!
I can't tell you how many people have said to me, "my cat is pooping outside their litter box." I say, "is it clean?" Nine times out of ten their eyes look down. If your litter is nice and clean, if the boxes are big and in a safe and quiet place for kitty's privacy and there are still potty problems, it is time to get your cat to your veterinarian.