I think I would feel nervous, too. My T does not have a supervisor so I have not been in this situation, but I believe I can understand why you'd feel uncomfortable, especially hearing what this other person says about you... I don't think you need to feel that the supervisor knows everything about you, though. There are lots of things that your T knows that she won't tell her supervisor, I'm sure. And wouldn't it be possible for you to find out a little bit about this person? Or would that break confidentiality rules?