Have your cats been to the vet lately to make sure there is no medical reason for this? Since you don't know which one is doing it, you need to address the problem with both cats. Once you know nothing is medically wrong, I would then examine my setup; how many boxes do you currently have? Where are the litterboxes located? Is one cat maybe bullying the other and denying access to the choice litterbox? Is one box favored over another? stuff like that.
The last time we had a problem with a cat going outside the box, it was Sasha, our tom. He started first going in the tub and then on the rug in the hallway. Long story short, after the vet ran the urinalysis and found that there were no problems there, it turned out Sasha just wanted his own box. He is very territorial. We haven't had that problem in over a year, but we had to add another littebox so now each cat has one they prefer. The thing is to catch it early enough and intervene before it becomes a bad habit. If allowed to go on too long, it's almost impossible to reverse the behavior.
Last edited by marie73; 01-03-2013 at 12:24 AM.