This wouldn't be a hard thing to implement but right now you'll need to get you hands dirty a little. Theoretically you'll need to run 2 x servers, what that is for LinuxMCE and uses the composite/svideo output from the video card and one x server running on the dvi connected display.
The linuxmce interface is using a different X11 server than the ubuntu/kubuntu display (btw never tested with kubuntu, you might need to install some dependecies for the installer before running it).
The configuration file for the LinuxMCE x11 is /etc/X11/xorg.conf.pluto and the one for your kde/gnome display should be /etc/X11/xorg.conf . So you'll need to tweak the xorg.conf.pluto one to only use the composite/s-video output. The bad part is that the xorg.conf.pluto file is regenerated from time to time so you'll need to hack into the scrips to make it permanent.
If you get it going you might write a wiki page about it