xrandr: screen cannot be larger than 1680x1680 (desired size 3360x1050)

xrandr: screen cannot be larger than 1680x1680 (desired size 3360x1050)

When executing this list of commands:
xrandr --output DFP3 --mode 1680x1050 --rate 60
xrandr --output CRT1 --mode 1680x1050 --rate 59
xrandr --output DFP3 --left-of CRT1 (error here)
xrandr --output CRT1 --primary
I have this output:
xrandr: screen cannot be larger than 1680x1680 (desired size 3360x1050)
I looked up and they recommended me to add a line in xorg.conf adding Virtual 3360x1050 to the Display subsection. I added it and made a reboot but I have the same error. Here is my xorg.conf:
Section "ServerLayout"
    Identifier     "aticonfig Layout"
    Screen      0  "aticonfig-Screen[0]-0" 0 0
EndSection

Section "Module"
EndSection

Section "Monitor"
    Identifier   "aticonfig-Monitor[0]-0"
    Option      "VendorName" "ATI Proprietary Driver"
    Option      "ModelName" "Generic Autodetecting Monitor"
    Option      "DPMS" "true"
EndSection

Section "Device"
    Identifier  "aticonfig-Device[0]-0"
    Driver      "fglrx"
    BusID       "PCI:1:0:0"
EndSection

Section "Screen"
    Identifier "aticonfig-Screen[0]-0"
    Device     "aticonfig-Device[0]-0"
    Monitor    "aticonfig-Monitor[0]-0"
    DefaultDepth     24
    SubSection "Display"
        Virtual 3360 1050
        Viewport   0 0
        Depth     24
    EndSubSection
EndSection