KDE: Dual DP LG Ultrafine 5k - as single screen

This is a follow-up to this post where @megavolt said that enabling Xinerama will enable KDE to treat two screens as one.

I have the same monitor as @kansas.manjar0 (LG Ultrafine 5k), and even have Xinerama enabled in Clear Linux:

$ xdpyinfo -ext XINERAMA  | tail -2
XINERAMA version 1.1 opcode: 140
  head #0: 5120x2880 @ 0,0

But KDE doesn’t recognize this has one unified screen. Does anyone know how I can diagnose this further?

I have long given up on this one. It just doesn’t work as the single steam monitor under KDE. It is always seen as two monitors.
I didn’t manage to use XINERAMA to achieve that.