40-monitor.conf

# cat /etc/X11/xorg.conf.d/40-monitor.conf

Section "Device"
  Identifier "Onboard Radeon"
  Driver     "radeon"
  BusID      "PCI:1:5:0"
  Option     "Monitor-HDMI-0" "Right screen"
EndSection

Section "Device"
  Identifier "PCI nVidia"
  Driver     "nvidia"
  BusID      "PCI:2:0:0"
  Option     "Monitor-DVI-0" "Left screen"
  Option     "Monitor-DVI-1" "Center screen"
EndSection

Section "Monitor"
  Identifier "Left screen"
#   Option     "Left screen" "LeftOf" "Monitor-HDMI-0"
EndSection

Section "Monitor"
  Identifier "Center screen"
  Option     "RightOf" "Left screen"
EndSection
Section "Device"
  Identifier "PCI nVidia"
  Driver     "nvidia"
  BusID      "PCI:2:0:0"
  Option     "Monitor-DVI-0" "Left screen"
  Option     "Monitor-DVI-1" "Center screen"
EndSection

Section "Monitor"
  Identifier "Left screen"
#   Option     "Left screen" "LeftOf" "Monitor-HDMI-0"
EndSection

Section "Monitor"
  Identifier "Right screen"
EndSection

Section "Screen"
  Identifier "Screen 0"
  Device     "Onboard Radeon"
  Monitor    "Right screen"
  DefaultDepth  24
  SubSection     "Display"
        Depth       24
  EndSubSection
EndSection

Section "Screen"
  Identifier "Screen 1"
  Device     "PCI nVidia"
  Monitor    "Left screen"
  DefaultDepth  24
  Option         "TwinView" "1"
  Option         "metamodes" "DFP: 1600x900_60 +0+0, CRT: 1680x1050_60 +1600+0"
  SubSection     "Display"
        Depth       24
  EndSubSection
EndSection