I think it depends on what kind of entertainment you have in mind. If it's just movies, one pc with two sound cards (or two video cards, depending on the board) should be enough. Now if each screen are going to run a videogame, then yes, it's better to have two pcs.
Note that if one screen is browsing the web while the other is playing video, each time the guy playing video touch the screen (say FF a video), it will steal focus from the other screen. The mouse pointer will be gone, until the screen is touched again. Not a big problem, but the other guy/kid may not expect that. If he is using a mouse, though, he may click somewhere on the video screen no knowing what he is doing, etc.