Scrivener for macOS and Windows allows you to view (and edit) two documents right alongside one another. You might have your writing on one side and a photo or PDF document to which you need to refer on the other, or you might be checking a previous or later chapter right alongside the one you are currently writing.
Did I say “Scrivener for macOS and Windows”? Well, now we can add the iPad to this list.
On the iPad, this is done using the Quick Reference feature: simply swipe left on a document in the binder, tap the “More” button, and then select “Quick Reference”. The document you swiped slides into view right there in the sidebar, replacing the binder, so that you have it alongside your writing. (You can easily make the sidebar wider to see more of the document, too.)
When you have the corkboard in the main editor, you can swipe left on a card to “throw” its document into the sidebar as a Quick Reference document. This is not only incredibly useful, but also quite a bit too much fun. (Though perhaps I should get out more.)