I feel for you.. cat pee is caustic and next to impossible to remove.
My cat Max, aka Floorpoop, just crapped in the fireplace and peed all over a nice sheepskin that ds and I sit on when reading. He has a history of 'freeranging' all over the basement, with no health issues (and a clean litterbox too!!), so we've decided to fence in the yard and kick him out. It's just too nasty and this has gone on for 6(!) years, and I've had it. There are enough nice cats in the world that can use a litterbox that I'm not putting up with it anymore. I hate him. DH is more compassionate and knows the shelters would put him to sleep and no friends want him either. We're building him a nice, safe place to live in the yard shed.
I hope your kitties knock it off soon! I find that intolerable!!