I’ve done a fresh install of Kodi 19.3 on my Raspberry Pi 4.
The HDMI0 (main) output is connected to a video projector, but I wanted to mirror the video output to the HDMI1 (secondary) port, that is connected to a small display.
The idea is that I won’t need to turn on the video projector every time I want to see the user interface.
Thanks Smooney, but this was not the goal - the secondary display was to avoid having to turn on the projector, since it needs a warm-up time before the image can be seen clearly. VNC replaces that with the need to wait for the client PC to boot.