essaie avec l´option xinerama dans le fichier xf86config ; )
avec un man XF86Config-4
on trouve entre autre :
dans
SERVERFLAGS SECTION
The ServerFlags section is used to specify some global X server
options. All of the entries in this section are Options, although for
compatibility purposes some of the old style entries are still recog-
nised. Those old style entries are not documented here, and using them
is discouraged.
Option " Xinerama" " boolean"
enable or disable XINERAMA extension. Default is disabled.