When a colleague and I team-taught our 9th-grade math classes for three years, we found out early on that we needed a way to make the presentations accessible to all students. The document camera and projector were an improvement over the whiteboard (easier to read, but also easier to show students’ work and have them present). But in our double-long classroom what we really needed was to synchronize our projectors. Just running a VGA cable wouldn’t work by itself, since we needed a way to switch sending/receiving roles and switch between camera and computer display. The solution was a VGA splitter box – we got two of these, each with two inputs and four outputs.
The outputs I have shown on mine are monitor (my screen), projector, 283 (the room next door), and student (another monitor I set up for near-sighted students). The student monitor ended up being highly desirable (students seemed to like having their own personal screen), to the point where I installed a second one of those.