This can be done in XRandR 1.5 without additional software albeit it's undocumented at the moment (at least I could not find a documentation for this). While neither fakexinerama nor FakeXRandR have worked for me on Xubuntu, this solution has finally split the screen into two.
To split the monitor do the following:
Enter xrandr
into the terminal to check the output name and current resolution of the display you want to split.
On my system the result was:
Screen 0: minimum 8 x 8, current 1920 x 1080, maximum 32767 x 32767
HDMI1 disconnected (normal left inverted right x axis y axis)
HDMI2 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 509mm x 286mm
1920x1080 60.00*+ 50.00 59.94
1920x1080i 60.00 50.00 59.94
1600x900 60.00
1280x1024 75.02 60.02
1152x864 75.00
1280x720 60.00 50.00 59.94
1024x768 75.08 60.00
800x600 75.00 60.32
720x576 50.00
720x576i 50.00
720x480 60.00 59.94
720x480i 60.00 59.94
640x480 75.00 60.00 59.94
720x400 70.08
VGA1 disconnected (normal left inverted right x axis y axis)
VIRTUAL1 disconnected (normal left inverted right x axis y axis)
We can see that my monitor is connected to HDMI2 and the resolution is set to 1920x1080.
Add 2 virtual monitors with the xrandr --setmonitor
command ensuring that they overlap with your physical display and are placed next to each other. The syntax of the command is (without quotes):
xrandr --setmonitor "monitor_name" "width_px"/"width_mm"x"height_px"/"height_mm"+"x_offset_px"+"y_offset_px" "output_name"
For my system it was:
xrandr --setmonitor HDMI2~1 960/254x1080/286+0+0 HDMI2
xrandr --setmonitor HDMI2~2 960/255x1080/286+960+0 none
While the above has already configured the virtual monitors on my system the changes haven't been applied until I have executed (it seems to refresh xrandr):
xrandr --fb 1921x1080
xrandr --fb 1920x1080
To persist the changes after reboot you would want to execute these commands when you login. You can do so by appending the commands at the end of your ~/.profile
file.