Matthew Modine in duel role

I have not seen the movie, but according to the cast listing and other places I've read about the movie, Matthew Modine fully plays both parts. It was probably cast that way to mirror of the connection made between the two characters in the book: Chipper Dove (as he was called in the book) was the quarterback of the Dairy School football team, while Ernst is called the "quarterback" of the communist radicals in Vienna. According to the narrator, Franny is attracted to the "cocksure quality" and "touch of evil" both of them possess. More to the point, they are said to resemble each other.