At the Adelphi University School of Social Work, full time professors are required to have a Ph.D, but most adjunct professors do not have a Ph.D. Some people are simultaneously adjunct professors with a Masters degree and Ph.D students at the same school. Is that rare?
I'm finding it less and less rare. Once you add in the idea of someone being a PhD student at one school, and professor at another, you run into a rapidly growing group (at least from my experience). In my 8 person PhD cohort we have 1 full time professor at another school here in Honolulu, a full time professor at another department at UH, a full time instructional designer at UH that also teaches a few classes in our department, and I'll be teaching my first class at UH this spring in a completely different department. That's a 50% ratio of people at least teaching something, and 25% ratio of those being full time professors. Going farther into my department alone, with a fairly small number of total PhD students (around 40 right now), you'll find several others doing the same types of things. Once you expand to our entire College of Education, PhD students who are at least adjuncts are found everywhere on campus. I feel like that might be a bit unique in Hawaii, with the very different circumstances geographically for us, but I've noticed it becoming more common when I meet people from other universities at various conferences.
Just finished teaching my first class (with a 4.63/5 rating from my students) and I've already been given a class for the fall and been asked to redesign and teach the class I just finished again next spring. Also have 2 class to go before hitting comps/proposals unless my program dumps the dissertation requirement.