kitty walker:  I can't believe you are contributing to the growing feral cat problem.  Just b/c there is a stray cat doesn't mean its ok for you to add to that.  You are risking your cats dying a slow and painful death.  You said in your last post that they come back to the house crying.  They will get their asses kicked by other stray animals.  Some people shoot or poison strays. 

OP: You didn't answer my other two questions.  Have you tried isolating them from each other and how often do you clean the litter box?  Talk to your vet.  They might be able to help.