I have two manual written by separate authors which I want to develop into one document after editing.
There is no MDI-like mode, and presently also not a window splitter. (There is an old feature request with this respect.) Anyway each document has its own window (frame) which can independently be sized and positioned. Thus: Simply display the two document-windows side by side on your screen. You can also have one document in more than one windows at the same time to edit/compare different regions in parallel: > ‘Window’ > ‘New Window’.