Hi I know this is out of the blue. I have been trying to set this up under slackware version 14.0 on two identical -- but old -- boxes. I was having the same problem, the mouse pointer would jiggle around in the margin of the second screen but never make it into the interior. It worked fine on the first screen though.
I noticed on a few howtos based on Debian and Ubuntu that they recommended getting hold of slightly older Debian etch packages since current versions of xdmx "didn't work", e.g.
So I actually downgraded to Slackware 13.37 and sure enough the problem went away. I guess there is just some problem in recent versions of xdmx. If you haven't already done so I would recommend trying the etch package: http://packages.debian.org/etch/xdmx
...if this is still an open problem that is!!