but it doesn't seem to change anything.
$ apt-cache search xinerama
libdmx-dev - X11 Distributed Multihead extension library (development headers)
libdmx1-dbg - X11 Distributed Multihead library (debug package)
libxcb-xinerama0 - X C Binding, xinerama extension
libxcb-xinerama0-dbg - X C Binding, xinerama extension, debugging symbols
libxcb-xinerama0-dev - X C Binding, xinerama extension, development files
libxinerama-dev - X11 Xinerama extension library (development headers)
libxinerama1-dbg - X11 Xinerama extension library (debug package)
nitrogen - wallpaper browser and changing utility for X
oroborus - A lightweight themeable windowmanager for X
pekwm - very light window manager
stumpwm - tiling, keyboard driven Common Lisp window manager
x11proto-xinerama-dev - X11 Xinerama extension wire protocol
xdmx - distributed multihead X server
libdmx1 - bibliothèque DMX (système distribué) pour X11
libxinerama1 - Bibliothèque de l'extension Xinerama pour X11
openbox - Gestionnaire de fenêtres rapide, léger, extensible et respectant les standards
totem - simple lecteur de media pour bureau GNOME basé sur GStreamer
xfdesktop4 - arrière-plan, icônes et gestionnaire de menu pour le bureau XFCE