A masters is a prerequisite for a PhD in every doctoral program I'm aware of. And it's not uncommon for students to get their advanced degrees from different institutions depending on what quality of program. And advanced degrees in different disciplines that related. Tyson got a masters in Astronomy from UT, which has a very good astronomy program. He got a masters and doctorate in Astrophysics from Columbia, which is one of the top schools in that department.
UT dissolved his doctoral committee. What does that tell you? He was dancing more than hitting the books. Later he got it together on the Upper West Side.
Depends on the school.