Sounds like territory marking! One of them wanted the box to themselves. If you put out another box, you might want to put out two. One for each of them. Also some items just call for a cat to pee in them. My indoor only cat, Jake used to pee on anything vinyl. He ruined cat furniture and bean bag chairs. It was just something about the material. If they seem to enjoy the cat nip, use a catnip mat instead. I have 2 from Target that are scratchy mats that I sprinkle cat nip on. I have to have two though or one of the cats will claim it.
Such good news on Roxie allowing more pets. Also great that she comes right back out after others leave. The stray I brought into the house in May, (Marvin) always hides when others come over, when he hears snowblowers or lawnmowers and also when we use the sweeper. He used to stay hidden for hours afterward. Now as soon as the noise stops or people leave, I call to him and he comes prancing out. He will never be a social cat with others and might always be afraid of loud noises, but he has learned that I let him know when it is safe to come back out.
