I know that frustration! We had three female cats (all spayed) and then this male stray, Tom, comes along and, well, he had some tomcat war wounds and I felt sorry for him and you know where this is going... we took him to the vet, got him treated and neutered, brought him home and...
Midnight, the alpha cat at the time, HATED him. In truth, she hates all male cats, because once upon a time she got knocked up and nearly died trying to feed her five kittens. (Someone took her to the SPCA, the kittens went into foster care, and we adopted her.) But she's had it in for males ever since.
So, she and Tom - we think it was those two - got into the whole peeing thing. On rugs. On the couch. On an empty pedestal cake plate. (Why??) On the pillow on my bed. In our shoes. Geez!
We decided that we had to lock Tom in the cellar. They simply had to be separated. In the end, my Mom adopted Tom and he never once did anything inappropriate in her house. But, he's the only cat there...
As to figuring out who did the peeing... bring in the innocent and/or guilty parties, one at a time, and out of sight of the others. The innocent will sniff it out, because they want to know what happened. The guilty will show no interest, because they already know about it.
The best cleanup product we ever found was "Stink Free Stain and Odor Urine Remover".
Good luck! It's maddening, isn't it?