Gary, I am not sure exactly what you are trying to do. Are you using Moodle's "group" features? Awhile ago, I had two sections of a course and I wanted to combine them into one Moodle course.  However, I wanted to keep their discussion separate.  I accomplished this by using groups.  It was a little tricky to set up.  I added another "me" user and had to modify the role a little, but it worked.  Students in one group did not see posts from students in the other group.