Indeed, MuPAD does not have the ability to show the content of the same document twice. Neither in one windows nor in separate ones. However, I agree that this would be helpful feature.
I use a real copy of the document for this purpose. I use the copy for viewing in one window and the original for editing in another one. Obviously, this is not a perfect solution, you have to take care of keeping them in sync. However, it helps a bit to avoid heavy scrolling.