At my wits end with peeing problems

I have two cats whom I love to death, but just for some reason dont pee in their litter boxes and it's about to push me to put them up for adoption. I never catch them doing it, I'll just randomly find a spot on a sheet or on my couch or floor where they've peed. I've read the sticky post and I've taken them to be checked out for urinary tract infections and they're both fine. The litter boxes are kept clean, I reward them when they use the litterbox but still I find these "surprises" every few days. I'm really at a loss with it and it's causing problems for my roomate as well, so I need to get this problem solved one way or the other, as much as I would hate that. Help!

peeing out of the litter boxes is either a medial problem or behavioural. If behavioural, then it's usually due to stress. Make sure you have more than one tray, i read somewhere that they allways suggest 3 trays for 2 cats. This is because one cat may be feeling intimidated using the other cats tray and stepping on their territory.