Hi @Albert2 . Welcome to the forums! 🙂
I'm no lawyer, so I can't attest to the accuracy of what I have to say. But my understanding is that, in the eyes of the law, you and all your alters are classified as just one person. I haven't heard anything about the laws of doctor-patient privelage specifically forbidding a doctor from disclosing patient conversations to other alters in the same body. I suspect that this is probably such a niche issue, the law hasn't ever stopped to think about drawing up clear standards on it.
My understanding is also that a person always retains the right to see their own file that their doctor/therapist keeps on them. Since, legally, you classify as a single person, I would assume that legally, all the alters in your body retain a legal right to see that file, and hence find out what all the other alters have been saying in therapy.