Hi,
I have same situation with only two displays.
xdmx start string:
====================
startx /usr/bin/openbox-session -- /usr/bin/Xdmx :1 \
-display :0\
-display :10\
-input :0 \
-fontpath built-ins \
-norender \
+xinerama
====================
tested
xdmx_1.10.4-1~bpo60+1
from squeeze-backports it's ok.
С уважением, Спицын А.А.
Best regards, Spitsyn A.A.