This problem is solved in Xorg just above the driver abstraction level.
RandR 1.5 addressed the problem of composite screens with a concept of a ‘Monitor’ back in March-2015.
https://cgit.freedesktop.org/xorg/proto/randrproto/tree/randrproto.txt?id=randrproto-1.5.0
man xrandr does not document ‘Monitor’-related options yet; See
xrandr --help
...
--listmonitors
--listactivemonitors
--setmonitor <name> {auto|<w>/<mmw>x<h>/<mmh>+<x>+<y>} {none|<output>,<output>,...}
--delmonitor <name>
I have two K1200 cards and setup two Xorg ‘screens’ in my xorg.conf, then combine separate tiles of each UP2715K into a named ‘Monitor’ in my .xininitrc:
xrandr --screen 0 --output DP-5 --mode 2560x2880 --pos 0x0 --rotate right
xrandr --screen 0 --output DP-4 --mode 2560x2880 --pos 0x2560 --rotate right
xrandr --screen 0 --output DP-6 --mode 2560x2880 --pos 2880x2560 --rotate right
xrandr --screen 0 --output DP-7 --mode 2560x2880 --pos 2880x0 --rotate right
xrandr --screen 0 --setmonitor DellUP2715Kmws0 2880/340x5120/600+0+0 DP-4,DP-5
xrandr --screen 0 --setmonitor DellUP2715Kmws1 2880/340x5120/600+2880+0 DP-6,DP-7
xrandr --screen 1 --output DP-5 --mode 2560x2880 --pos 0x0 --rotate right
xrandr --screen 1 --output DP-4 --mode 2560x2880 --pos 0x2560 --rotate right
...
DISPLAY=:0.1 exec i3 &
DISPLAY=:0.0 exec i3
i3 WM implemented support for Randr 1.5 Nov-2016, it is not yet in current (8-Nov-2016) releases but is in the NEXT git branch. It works and is amazing. Base Mosaic doesn’t work for me in this setup so there are two independent xorg ‘screens’. NVIDIA?
Thanks and bravo to Jim Gettys and Keith Packard (RandR 1.5) and Michael Stapelberg (i3wm).