I have a Carbon X1 running Ubuntu 19.04 with KDE Plasma 5. When colleagues also running KDE plug an external monitor in, System Settings -> Display and Monitor will show all the displays. Mine however only show one box, even though the Primary Display dropdown lists No Primary Output, Laptop Screen and U28D590 (HDM1). This seems to mean I can mirror my output on both screens, but nothing else.
How do I get both displays to appear, so I can arrange them, and display different out on each.