As far as I know this is not possible, but there is a workaround that you could use
- create your landscape as one single part
- edit it
- when you are finished, open the gizmo tool and export the heightmap of the first part + after you did it, delete the copied part and save your level
- open a new level - go to the landscape tab - import the heightmap - save the level
or do it this way: ^^