LinuxMCE does not currently have any built-in support for multiple displays. You must have a separate MD for each display.
Is there not some way he could clone the display? Essentially have the video output on both the monitor and projector at the same time?
The simplest way would be to use a video splitter on the MD's output and use the same connector type & resolution on both the normal TV display and the projector. Then use rs232 or IR to power off/on the display that is not in use when appropriate.
All the best