I'm so sorry to hear this. I went through something like this in the summer of 2007. Luckily I knew that it was Storm and Starr that were doing it so I had them checkd out at the vet. They were fine so my vet told me to buy some Cat Attract Litter by Dr. Elsey and you can buy it at Petsmart. It also has a great booklet inside that tells you everything you need to know about inappropriate peeing.

I've also been using the Feliway/Comfort Zone plugins and they've helped calm down my cats. Storm is also on calming herbal pills. I also added 3 more litter boxes so I now have 8 litter boxes for 6 cats but they usually only use 4-5 of them. Good luck and I hope that you'll be able to solve this problem quickly.