Soiriane's response may be what you need. You may have to do some arranging.

Word, dating back to the days before Windows, used to be able to open two (or more) documents in one window, with a single set of menus on top. The method recommended gives two windows, each with its own Ribbon.

I believe that there are third-party applications that do it all in one. I know of no way native to Word to do this in recent versions.