A cat question for my readers...

Kelly got home today and discovered that one of the cats had decided to pee on our bed. This is unprecedented in the Slinkerton household. Since Kelly has had Maura for over 10 years, the logical suspect is Brando. He stayed hidden under the bed until I got home. We're not sure what's going on. He and Maura have been using the same litterbox for the last few days and have seemed to be doing okay. They're not fighting and they seem to be okay being in the same room together. Brando is still very skittish and will run away if Kelly or I walk towards him, but if we sit on the floor he will come hang out with us when he wants attention. He's a lot less skittish than he was a few weeks ago and we don't think he's unhappy enough that we should return him to the shelter but it's not like we can really tell. Any of you with multiple cats have any tips for dealing with this kind of situation?

Cat update

Turns out that today's cat incident was all Kelly's fault. When she changed the kitty litter, she accidentally left the box positioned so that the open side was facing the wall. She felt pretty dumb when she figured that out.