Separate X11 screens on dual-head Raspberry Pi

xorg.conf

# Sets up Xorg on Raspberry Pi 3b in FKMS mode for two screens.                                 
#                                                                                                      
# To put the Pi into FKMS mode, add one of the following lines to /boot/config.txt:                  
#  1. "fake" KMS mode (exposes the proprietary vc4 firmware driver via KMS API):                       
#        dtoverlay=vc4-fkms-v3d                                                                        
#  If you wanted "full" KMS mode, fully open source "vc4" driver in the Linux kernel, you
#  would use `dtoverlay=vc4-kms-v3d` instead, but then the identifiers in the Driver section
#  need to be different.
#                                                                                                      
# To run X clients on one screen or another:                                                           
#   Address the DPI display with DISPLAY=:0.0                                                          
#   Address the HDMI display with DISPLAY=:0.1                                                         

Section "Device"
    Identifier      "VideoCore4-DPI"
    Driver          "modesetting"

    # this allows the HDMI device to get page flipping priority over DPI.
    # Without this option, you will experience tearing on every HDMI frame
    # plus a stuttering update cadence on HDMI leading to a very unstable
    # frame rate.
    Option "PageFlip" "false"

    Screen          0
EndSection

Section "Device"
    Identifier      "VideoCore4-HDMI"
    Driver          "modesetting"
    Option "PageFlip" "true"
    Screen          1
EndSection

Section "Monitor"
    Identifier      "HDMI-1"
    Option          "Primary" "true"
EndSection

Section "Monitor"
    Identifier      "DPI-1"
    Option          "Primary" "false"
    Option          "Rotate" "normal"
EndSection

Section "Screen"
    Identifier      "DPI Screen"
    Device          "VideoCore4-DPI"
    Monitor         "DPI-1"
    DefaultDepth    24
        SubSection "Display"
            Depth       24
            Modes       "FIXED_MODE"
    EndSubSection
EndSection

Section "Screen"
    Identifier      "HDMI Screen"
    Device          "VideoCore4-HDMI"
    Monitor         "HDMI-1"
    DefaultDepth    24
        SubSection "Display"
            Depth   24
            Modes   "1280x720"
        EndSubSection
EndSection

Section "ServerLayout"
    Identifier      "SeparateScreens"
    Screen 0        "DPI Screen"
    Screen 1        "HDMI Screen" RightOf "DPI Screen"
EndSection

Section "ServerFlags"
    # These options disable screen blanking, which is important for embedded systems
    Option "BlankTime"    "0"
    Option "StandbyTime"  "0"
    Option "SuspendTime"  "0"
    Option "OffTime"      "0"
EndSection