How can I get xrandr to detect both nvidia cards: 2 x GTX480, triple-head?

Martin- Very similar setup on my end, except with a pair of nvidia GT9800's - circa 2008!. Also an onboard Intel HD4000 (disabled in bios). xrandr only showed 1 gpu, though all other sys related tools properly reported both. ubuntu 14.04 lts beta 2, nvidia 331.28 proprietary

The holy grail fix for me last night was: Base Mosaic! Empty xorg.conf, nvidia x server settings-> Enable Base Mosaic, then enable each detected display. It all just works! fast seemless multi-gpu, multi-head multi-monitor setup with 1 xscreen! My 3 displays are different resolutions and one is rotated portrait. Even the ubuntu Settings - Displays correctly shows all 3 monitors.

I suggest upgrading to the nvidia 331.28 proprietary drivers, blow out your xorg.conf, and try base Mosaic again.

-chad

➜ ~ nvidia-xconfig --query-gpu-info

Number of GPUs: 2
GPU #0:
  Name      : GeForce 9800 GT
  UUID      : GPU-56281125-01c7-ee85-b8b9-b3e5cf94b7c5
  PCI BusID : PCI:1:0:0
  Number of Display Devices: 1
  Display Device 0 (CRT-0):
      EDID Name             : Samsung SyncMaster
      Minimum HorizSync     : 30.000 kHz
      Maximum HorizSync     : 81.000 kHz
      Minimum VertRefresh   : 56 Hz
      Maximum VertRefresh   : 75 Hz
      Maximum PixelClock    : 146.250 MHz
      Maximum Width         : 1680 pixels
      Maximum Height        : 1050 pixels
      Preferred Width       : 1680 pixels
      Preferred Height      : 1050 pixels
      Preferred VertRefresh : 60 Hz
      Physical Width        : 470 mm
      Physical Height       : 300 mm
GPU #1:
  Name      : GeForce 9800 GT
  UUID      : GPU-cbc59e3f-e8c1-a347-a069-b62f6b885f7d
  PCI BusID : PCI:2:0:0
  Number of Display Devices: 2
  Display Device 0 (CRT-0):
      EDID Name             : Samsung SyncMaster
      Minimum HorizSync     : 30.000 kHz
      Maximum HorizSync     : 81.000 kHz
      Minimum VertRefresh   : 56 Hz
      Maximum VertRefresh   : 75 Hz
      Maximum PixelClock    : 140.000 MHz
      Maximum Width         : 1280 pixels
      Maximum Height        : 1024 pixels
      Preferred Width       : 1440 pixels
      Preferred Height      : 900 pixels
      Preferred VertRefresh : 60 Hz
      Physical Width        : 410 mm
      Physical Height       : 270 mm

  Display Device 1 (DFP-1):
      EDID Name             : Ancor Communications Inc ASUS VH242H
      Minimum HorizSync     : 30.000 kHz
      Maximum HorizSync     : 85.000 kHz
      Minimum VertRefresh   : 55 Hz
      Maximum VertRefresh   : 75 Hz
      Maximum PixelClock    : 160.000 MHz
      Maximum Width         : 1920 pixels
      Maximum Height        : 1080 pixels
      Preferred Width       : 1920 pixels
      Preferred Height      : 1080 pixels
      Preferred VertRefresh : 60 Hz
      Physical Width        : 520 mm
      Physical Height       : 290 mm

Below is my xorg.conf - All of which x server settings generated. Note that it is quite straight forward, and does not even reference my second GPU.

# nvidia-settings: X configuration file generated by nvidia-settings
# nvidia-settings:  version 331.20  (buildd@roseapple)  Mon Feb  3 15:07:22 UTC 2014

Section "ServerLayout"
    Identifier     "Layout0"
    Screen      0  "Screen0" 0 0
    InputDevice    "Keyboard0" "CoreKeyboard"
    InputDevice    "Mouse0" "CorePointer"
    Option         "Xinerama" "0"
EndSection

Section "Files"
EndSection

Section "InputDevice"

    # generated from default
    Identifier     "Mouse0"
    Driver         "mouse"
    Option         "Protocol" "auto"
    Option         "Device" "/dev/psaux"
    Option         "Emulate3Buttons" "no"
    Option         "ZAxisMapping" "4 5"
EndSection

Section "InputDevice"

    # generated from default
    Identifier     "Keyboard0"
    Driver         "kbd"
EndSection

Section "Monitor"

    # HorizSync source: edid, VertRefresh source: edid
    Identifier     "Monitor0"
    VendorName     "Unknown"
    ModelName      "Samsung SyncMaster"
    HorizSync       30.0 - 81.0
    VertRefresh     56.0 - 75.0
    Option         "DPMS"
EndSection

Section "Device"
    Identifier     "Device0"
    Driver         "nvidia"
    VendorName     "NVIDIA Corporation"
    BoardName      "GeForce 9800 GT"
    BusID          "PCI:1:0:0"
EndSection

Section "Screen"

auto-select +3600+0 {rotation=right}, GPU-cbc59e3f-e8c1-a347-a069-b62f6b885f7d.GPU-1.DVI-I-3: nvidia-auto-select +1680+180"
    Identifier     "Screen0"
    Device         "Device0"
    Monitor        "Monitor0"
    DefaultDepth    24
    Option         "Stereo" "0"
    Option         "nvidiaXineramaInfoOrder" "CRT-0"
    Option         "metamodes" "GPU-56281125-01c7-ee85-b8b9-b3e5cf94b7c5.GPU-0.DVI-I-0: nvidia-auto-select +0+180, GPU-cbc59e3f-e8c1-a347-a069-b62f6b885f7d.GPU-1.DVI-I-0: nvidia-auto-select +3600+0 {rotation=right}, GPU-cbc59e3f-e8c1-a347-a069-b62f6b885f7d.GPU-1.DVI-I-3: nvidia-auto-select +1680+180"
    Option         "MultiGPU" "Off"
    Option         "SLI" "off"
    Option         "BaseMosaic" "on"
    SubSection     "Display"
        Depth       24
    EndSubSection
EndSection