você pode usar o fakexinerama 'para alcançar o que deseja:
Fake Xinerama is a replacement libXinerama library that instead
of querying the XServer reads ~/.fakexinerama and provides fake
information about Xinerama screens based on this file. It can be
used to fake a Xinerama setup even on computers with just one monitor
or to fake Xinerama setup other than one specified in the XServer
configuration (e.g. making one screen smaller when using two same
screens). It's probably only useful for developers.