To answer another of your questions - can one MD control another MD to play some media?
Yes, but you should understand the infrastructure a little better first that makes this possible.
An MD (Media Director) is simply the station that media can be piped through to an Entertainment Area
An Orbiter is a control interface that is used to control media playback, HA, telecom, security, etc on any MD.
Every MD happens to have an "On Screen" Orbiter, however there are other types, Mobile Orbiter, Windows Orbiter, Java Orbiter, etc. All have in common that they can control any MD.
So it really isn't a case of one MD controlling another, it is just that you are using the On Screen Orbiter of one MD to control another MD, just as you could use any other Orbiter (say on a mobile phone, your Windows machine, Cisco IP phone, etc) to do the same thing.