[jltsiren]

The academia is far more diverse than people think. More diverse between countries, between universities, between disciplines, and over time.

Having a junior and a senior advisor is fairly common. Sometimes the work is done in multiple institutions, with a separate advisor in each. In some systems, most people who supervise students are not formally qualified to do so, creating a need for a separate formal supervisor. Sometimes there are two equal advisors, and sometimes the advisor changes for various reasons. Sometimes the student is an independent scholar and the advisors are only loosely involved in the work. If you only have written records, it can be impossible to tell which of these was the case for a particular student.