I know this post is a year old at this point, but I stumbled across it in a Google search and might as well answer it in case anyone else finds this thread.
Yes, you can have multiple landscape levels in World Composition and edit the landscape from each together as if they’re a single landscape basically making a custom tiled landscape you can edit by hand yourself. All without leaving the editor. It’s explained at the timestamped part of this live training stream Getting Started with World Composition | Live Training | Unreal Engine - YouTube
Basically all you need to do is make a level with a landscape in it. (I’m not sure if it’s allowed to have other objects in it or not) Then in the World Composition panel right click on the landscape level, scroll down to “Add Adjacent Landscape Level” and chose which side you want to add to. Then when you go to edit the landscape it will edit all of them as if they’re the same landscape.