the last time I had a problem with improper urination I suspected it was a new comer to the household distressing some one. I was wrong, it was a medical problem. Diabetes to be exact. The only way I could find out who was to isolate. Now Having 9 cats at the time and a 5 bedroom house I just put 2 in each and narrowed it down to one set of two and then separated them. This may be your only hope unless you can catch them in the act (got a video camera?) Use your insticts, they usually do not steer you wrong. I THOUGHT I knew who, and I turned out to be right.

If it is not a medical problem, try to figure out what could have changed that might be streesing one of them.