Apparently you need to add this to /etc/X11/xorg.conf in your wacom section :
And to restrict to a single screen :
Option "Twinview" "horizontal"
I don't have a wacom (yet) though, so I can't test it. I also use TwinView, and I'm hoping that when it arrives I won't run into any problems because of it. I'm also having the issue where some games will try to fullscreen across both monitors instead of (being correctly restricted to) the one I launched it in. =/ Fortunately I'm not big on gaming.
Option "ScreenNo" "0"