This is the last movement of the F minor quartet from Haydn's Op. 20 set. What's this? Not the jaunty classical Haydn you are used to? That's right. This is a double fugue. The 2nd violin presents the first subject, then the viola follows up with the second one. It's incredibly intellectual and exciting music.
In a "regular" fugue, each instrument enters with the melody (called the subject) one at a time. A series of episodes and returns to the subject follow, through different keys and variations. In this double fugue, there are two subjects - just to make it even cooler.