Further I tried using a minimalistic snippet, as suggested by @generix in this thread: Xorg configuration - two NVidia cards, xinerama breaks hardware acceleration, 390.25 current driver - #19 by sass.joel
That brought up a visible desktop across the two cards, but I could not operate it. I can do one right-click and the menu comes up and whatever I choose, nothing comes up and nothing responds anymore. I can move around the mouse-pointer, though.
I made sure that the BIOS addresses above 4GB.
I tried my vanilla-kernel as well as the Debian ones, again.