To be more specific, you can have as many media streams running (audio, video, Thom's Mame games, internet radio, etc) simultaneously as you have media directors. That's kind of the point of LMCE. And as Thom says, you can "move" a stream in real-time from one media director to another, or even "duplicate" a stream so that 2 or more media directors can be playing the same stream at the same time, in sync.
Also, the concept of the Orbiter is that it is a control interface either overlayed on top of your media "on screen" or on another device (PC, mobile phone, PDA, etc) that can control any media director in any entertainment area, and instruct any media director to pull media from any other location.
So, for instance, you can pop a DVD in the drive of one media director in your lounge room, go to the Orbiter menus on your bedroom (2nd) media director, and instruct the (3rd) media director in your family room to start playing the DVD in the drive in your lounge room! Then actually go into your lounge room and use that media director's Orbiter menus to "pull" that stream from your family room back into your lounge room - either by duplicating it, so it continues to play in the family room, or simply moving it so that the family room stops playing media, and after a short time, shuts down your entertainment equipment and turns off your TV