Option ProbeAllGpus in new X!11 xorg.conf for multiseat

Section "Device"
    Identifier     "device1"
    Driver         "nvidia"
    VendorName     "nVidia Corporation"
    BoardName      "NVIDIA GeForce 610 GT"
    BusID "PCI:1:0:0"
EndSection

Section "Device"
    Identifier     "device2"
    Driver         "nvidia"
    VendorName     "nVidia Corporation"
    BoardName      "NVIDIA GeForce 8400 GS"
    BusID "PCI:5:0:0"
   #required for multiseat, see bookmark in google
    Option "ProbeAllGpus" "false"
EndSection