I'm using the latest alpha of LinuxMCE.
Whenever I try to play from two sources (for example, a DVD on the core/hybrid and liveTV on a media director), the playback on either of the two freezes. I can go back to the main menu, shut off the playback, and then start it again from one of the media directors or the core/hybrid, but basically I can only play on one machine at a time.