Mine don't do this but I've heard from others that some cats prefer to pee in one box and poop in another. Since you don't have very many boxes and they poop in them then this may also be a reason why they're peeing outside of the litter boxes. Some cats are very picky and won't even use a box that others have used. I only have to scoop 2 times a day and mine hardly ever use all 8 boxes but they are there if they need them. I'd also rather put up with looking at and cleaning more boxes than having pee and urine odor throughout my place. I sure hope that something will work for you. Good luck.