Extend Linux Desktop to another X Windows Display
I think you are looking for http://dmx.sourceforge.net/ . From the man page:
It provides multi-head X functionality for displays that might be located on different machines.
Start the X server with the -ac option, in all Linux box that you want to make the wall, like:
X -ac
Then execute startx, with Xdmx with xinerama enabled, sending the display to the server, as:
startx -- /usr/bin/Xdmx :0 +xinerama -display 10.0.0.1:0.0 -display 10.0.0.2:0.0 -display 10.0.0.3:0.0 -norender -noglxproxy