It would be useful to have document view splitted in two (or more) views with indipendent page scrolling. Use case: you are reading for example, paragraph 5.2.2 and at certain point the book tells you to look again to 3.1.1.1 formula. You may want to be able to scroll to that page while keeping 5.2.2 paragraph in front of you.
*** This bug has been marked as a duplicate of bug 169847 ***