I use both an LCD 16:10 DVI connected screen and a Sanyo Z1 16:9 projector connected via RGB to watch recordings using MythTV (0.21-fixes). Until last week I had MythBuntu Hardy (8.04) running on my backend+frontend, but since 9.04 will be out in a few months I decided that the intermediate step to MythBuntu Intrepid (8.10) had to be made. So I upgraded, fought myself through some complications with the currently stable 180.22 nvidia driver for my 8600GT card and had KDE on the desktop for the first time (4.1.2 I think). To make the story short, the second screen had no desktop and would not start mythfrontend.
The solution was somewhat difficult to get to. Just using "export DISPLAY=:0.1" did not work, it would just told me that that screen was unavailable. So I had to redo my xorg.conf file to have both Xinerama and Twinview enabled using the nvidia-settings command but running it as root (sudo). After this, I found out, that just running mythfrontend would always open it on my larger LCD screen. It turns out that mythfrontend has a setting to choose which Xinerama screen it should use. That way it is possible to have mythfrontend open on the projector. Just make sure hat you also change the aspect ratio.
If you like to have these settings automatically changed when starting mythfrontend you need to tell it to change the above settings. I use the following command: "mythfrontend -O XineramaMonitorAspectRatio=1.6 -O XineramaScreen=0" to open on the LCD and "mythfrontend -O XineramaMonitorAspectRatio=1.7777 -O XineramaScreen=1" to start on the projector.