My first attempt at having the headrest screens for movies was using one PC with a VGA and a video out.
The video was supposed to play the movies.
However I just found it tricky to get it to work how I wanted.
None of the Frontends really handle multizone very well.
Also you would need two soundcards as well, so I am now moving to having a PC for the front and a second which will do the headrest monitors. (maybe one day if it all starts to work nicely I will use a third PC so the two headrest monitors can be individual.