Comment 22 for bug 240916

As a workaround, for those who can run command line tools, there's instructions how to generate the appropriate modelines and where to stick them in xorg.conf in a comment in the duplicate bug...
https://bugs.launchpad.net/ubuntu/+source/xorg-server/+bug/260436/comments/18
...which has apparently been useful to some.

I'll try to restate them more clearly here. They rely on two tools - gtf and xrandr - and a text editor.

xrandr should be invoked with no arguments to tell you the names of your displays.

These are the names which should be used in the xorg.conf later. Here's the output shown in my command line.

cefn@cefn-linux-tablet:~$ xrandr
Screen 0: minimum 320 x 200, current 1280 x 768, maximum 1440 x 1792
VGA disconnected (normal left inverted right x axis y axis)
LVDS connected 1280x768+0+0 (normal left inverted right x axis y axis) 0mm x 0mm
   1280x768_60.00 60.0*+
   1280x768 59.9 +
   1024x768 85.0 75.0 70.1 60.0
   1024x600_60.00 60.0
   832x624 74.6
   800x600 85.1 72.2 75.0 60.3 56.2
   640x480 85.0 72.8 75.0 59.9
   720x400 85.0
   640x400 85.1
   640x350 85.1
TV disconnected (normal left inverted right x axis y axis)

Then gtf can be invoked to create a modeline suitable for you based on what you know about your display, maybe from reading the manual.

cefn@cefn-linux-tablet:~$ gtf 1440 900 60

  # 1440x900 @ 60.00 Hz (GTF) hsync: 55.92 kHz; pclk: 106.47 MHz
  Modeline "1440x900_60.00" 106.47 1440 1520 1672 1904 900 901 904 932 -HSync +Vsync

Finally you should create an entry in your xorg.conf which combines all these bits of information. It's advisable to save a copy of your old xorg.conf with a new name, like xorg.conf.bak. In some cases your existing xorg.conf will be blank.

My modified xorg.conf looked as below. Note the declaration of a virtual resolution which was wide enough to put the displays side by side or above each other...

Section "Monitor"
 Identifier "Configured Monitor"
EndSection

Section "Monitor"
 Identifier "VGA"
 Modeline "1440x900_60.00" 106.47 1440 1520 1672 1904 900 901 904 932 -HSync +Vsync
EndSection

Section "Screen"
 Identifier "Default Screen"
 Monitor "Configured Monitor"
 Device "Configured Video Device"
 SubSection "Display"
  Virtual 1440 1700
 EndSubSection
EndSection

Section "Device"
 Identifier "Configured Video Device"
EndSection

Once you've made these edits, and you log out and log back in, the new modeline should be visible even in your graphical tool like grandr. If not, you can invoke the display changes using xrandr.