Hi Chandra:
Welcome to our little forum.
I guess the first thing I will ask is this: "Have you talked to your vet?". Before you rule out behavioral issues, you need to look and see if there is any underlying behavioral problems. Now, normally, it would be one cat having the issue, not both. However, since it sounds as if you were not sure if both are having the problem, it would be a good idea to talk to you vet and see if there is any underlying medical problem.
For two cats, you have the right number of litter boxes (1 per each cat, plus on extra). How often are the pans scooped? How often are the pans themselves cleaned? Has there been any changes recently? By changes, did you change the brand of litter, go from unscented to scented, to/from clumping, etc? Have the locations of the litter boxes changed? Were there any other changes in the household?
Is it possible for you to put a new litter box over where the accidents are occurring?
These are just some questions and thoughts. What exactly is the "no stay" stuff you are using?
I'm not sure I was able to help out much, but if you can give us some more information from the above questions, I'm sure one of the staff folks may be able to help.
Mark