For electric bass teaching, upright is unnecessary, though it doesn't hurt. But your teacher should really know something about playing electric bass.
Yes, someone can be a guitar player and a bass player and teach electric bass. I consider myself both a bassist and a guitarist, and I think I could teach either one about equally well. But some guitar teachers don't really play bass--they just figure it's easy because "the strings are the same as the lowest four strings of a guitar." I would tend to avoid those guys; by and large they can't help you much with playing the bass. Some of them do have a good understanding of music and theory in general, and so may have something to teach you in that regard, but I still would look for a real bass teacher if I were a beginner or intermediate student.