multi-head RPi on virtual screen

xorg.conf

Section "Monitor"
  Identifier "real_monitor"
EndSection

Section "Screen"
  Identifier "real_screen"
  # Device configured in /usr/share/X11/xorg.conf.d/99-fbturbo.conf
  Device     "Allwinner A10/A13 FBDEV"
  Monitor    "real_monitor"
EndSection

Section "Device"
  Identifier "dummy_videocard"
  Driver     "dummy"
  # VideoRam size in KiB.
  # 1024x768 24bpp would need 3072 KiB
  # Didn't check why it is 1024 * 768 * 4 instead of *3.
  # Since memory on RPi is tight, we should try to be parsimonious.
  # We should also try to be safe and give at least two buffers.
  VideoRam 8192
EndSection

Section "Monitor"
  Identifier "dummy_monitor"
EndSection

Section "Screen"
  Identifier "dummy_screen"
  Device     "dummy_videocard"
  Monitor    "dummy_monitor"
  SubSection "Display"
    Depth 24
    # Set the dummy screen resolution.
    Modes "1024x768"
  EndSubSection
EndSection

Section "ServerLayout"
  Identifier "XineramaLayout"
  # Screen 0 is the primary screen. Swap real_screen and dummy_screen to
  # change the primary.
  Screen     0 "real_screen" 0 0
  # Replace RightOf by LeftOf/Above/Below for different placement.
  Screen     1 "dummy_screen" RightOf "real_screen"
  # Extend display on both screens using Xinerama.
  Option     "Xinerama" "on"
EndSection