Originally Posted by kriko
Launching seat0, alltogether works. How can I start kde (startx) from console to use different credentials (different users) and different screens?
Of course you need to configure the X display manager to start a login on all your different Xephyr instances.
This is the GDM part of the instructions you referred to.
It can also be a matter of editing /etc/X11/xdm/Xservers
This is also required to associate the different keyboard and mouses with the correct screen.
(not really related to the NVIDIA driver)