That is the exact same thing I was thinking about last week! I would like the dash screen to show the GPS and be able to have master control over the back screens. The back screens could play a movie or show a visualisation of the mp3 while the front shows the controls or the GPS mapping.
The reason being of-course that I wouldn't want a friends episode being played while I was driving, too tempting to wonder why everyone around me is laughing and look at it.
No reason this can't be done.
I also wanted to support two sound cards, one for the music and one for the gps speech synthesised voice instruction directions. This I've got working I think.