I third the disease rule-outs. My buddy, Beau, a handsome tuxedo cat jumped up on the couch and let fly. The funny thing was the cat sitter had accused the *other* cat of doing it. Because we were gone, etc, we thought it would be acting out...but he was a diabetic and couldn't control himself.
He began to travel with us in our VW camper and got insulin shots 2 x a day. If we had not been with him all the time, he would not have lived as long as he did as he was a so-called brittle diabetic. He would suddenly have his pancreas kick in, for example. He lived a good long life for another 4 or 5 years. He was such a chow hound we waited till he began eating and could shoot him up. My dh got really great at it.
But we have also had a bunch of cats with urinary tract infections, it's really common esp. in male cats. Please do see about fixing them if they have not been, it may be a contributing factor. Also, have you changed your litter, cat food, water source? I have seen a recommendation for one more litter box then cats, but there was no way we could do that!
Good luck with your kitties, I know how hard it can be with problems with them. The vet recommended Feliway, but FON, and 75% water/25% vinegar have also worked. The key is to make sure the smell is gone.