> Similarly I bet you'll have to turn off some of the systemd magic so that you can do whatever you do to get multi-card Xinerama to work (I assume you write your own xorg.conf?).
Sure, I just hope they document how to demagic things. Not sure, but I expect there are a fair number of Xinerama users.
Yes, Xinerama is configured through xorg.conf. Essentially, you set up multiple "Screen"s and list their relative positions in "ServerLayout".