[JeromeK]

Hi,

I found the solution:

1. Remove all the closed source crap from display-link, including their kernel drivers.
2. Remove the udlfb driver from blacklist
3. Configure the udlfb modules to be loaded with the proper option (in /etc/modprobe.d)
4. ensure nouveau and udlfb are always loaded in the same order in /etc/modules
5. re-create the seat1 which points to the right /dev/fbX, i.e. the displaylink one
6. re-configure the /etc/X11/xorg.conf for the seat1/second layout with the fbdev driver pointing to the proper /dev/fbX

Apparently this stopped working with the newer USB3 display-link devices ... it is a bit sad as it is exactly MY use-case. At least I will not spend money on a new device