We are a two Mac family so we have two itunes libraries but just the one ATV. Is it possible to stream music to the ATV from both iTunes libraries?
you can sync up to 5 computers per ATV. not hard to do at all either. Just go into your mac or pc's itunes and go to preferences>sharing>share all. this will let you share movies and music, and photos(i think you can share photos as well). And yes you can have both mac and pc's connected to the same ATV. My wife has her PC and i have my macbook connected and it runs great.
did you define the other computer on the ATV? settings-> computers?
i don't think you need file sharing to be turned on for ATV.
also, you can only "sync" one computer. you can stream from 4 others.