Section "Device"
Identifier "Intel Graphics 0"
Driver "intel"
Option "ZaphodHeads" "HDMI1"
Option "AccelMethod" "sna"
Screen 0
EndSection
Section "Device"
Identifier "Intel Graphics 1"
Driver "intel"
Option "ZaphodHeads" "eDP1"
Option "AccelMethod" "sna"
Screen 1
EndSection
Section "Screen"
Identifier "Screen0"
Device "Intel Graphics 0"
EndSection
Section "Screen"
Identifier "Screen1"
Device "Intel Graphics 1"
EndSection
Section "ServerLayout"
Identifier "Double Layout"
Screen "Screen0" 0 0
Screen "Screen1" RightOf "Screen0"
EndSection
source: internet
basically you tell what device should use what driver and set screen layout...
probably won't work with systemd, since it's even bigger ass than xorg
tl;dr: pic related
