What do you mean by 'supervised solo'?
Normally only one person in an aircraft can be PIC. The only exceptions under JAA are SPIC on integrated courses and successful flight tests for rating issue where the instructor/examiner claims PIC and the student claims SPIC or PIC u/s respectively.
If you have a licence and someone else flys with you then you are PIC unless he has a licence and he claims PIC; in that case you can claim Pu/t if he's an instructor or nothing at all if he isn't.
It's all in LASORS