There are a few tutorials (for example this one, screen shots below) which show you can add a new pane to a workspace but in my version of Blender 2.8 (running on Windows 10) I can't recreate this option. It looks like the handle is missing. The handle being the little diagonal lines in the top right corner.
1 Answer
It appears that the UI is different? You can create a new editor pane inside a workspace by clicking and dragging in the tiny dark space between the panes. See annotated screenshot below.