thank you, "multiseat" was an excellent keyword for further research.
unfortunately, starting the second X server on the same card kills the first one in a bad way (no matter if NoInt10 is set or not), resulting in a garbled screen when you try to touch or exit the first server after the second started.
Is this a driver bug, an xorg bug or just a result of me shooting myself in the foot by using experimental configuration options (i.e. a bug noone cares to fix
As per comment 4.1
from Chris Tylers on the blog entry you linked, what I'm trying is not (yet) possible on a single GPU. I'll need to recalculate my power consumption and check if my PSU is strong enough before mounting the second card though, so I cannot yet report on any results.
Xephyr would work if it weren't for the fact that I need 3d acceleration. Honestly, the most frequent use case of this setup would be multiplayer gaming with friends. I don't need a second workstation, since noone else works here.