Hi. I have 2 boxes, one running 16.04 the other 18.04. Both connected to 2 iiyama E2482HD monitors (via kvm). Both setups arranged so I have 3840px wide dual-screen; both monitors appeared in both systems on installation (or connection in the case of 16.04 box) without me doing anything clever. Saturday I did a fresh install of 18.04 on the 'old' 16.04 box. It has an nvidia gpu and radeon pci-e card, and only the gpu-connected monitor appeared.
Following this thread, I established both were being detected.
The old 16.04 had no /etc/X11/xorg.conf but a ~/.config/monitors.xml file identifying 2 devices. Copied this across, reboot, no joy.
So I went into the pc bios and changed the video boot/preference order (ie. pci-e first). And both monitors are up and running.
Not sure if this helps, but it solved my issue