The most popular all-time etude collection for horn are the 60 Etudes (op. 6) by German hornist and composer Georg Kopprasch (c. 1800 - c. 1850). These etudes (composed around 1830), while being useful in the advancement of a horn player's technique, are not something anyone plays for fun. They are more mechanical than musical, and as such, are more like taking a not-very-pleasant medicine - it's good for you, but that is about the only attraction. Duet Kopprasch makes the original etudes more palatable - even (dare we say it) fun by adding a social dimension to the etudes, the same way that team sports make exercise much more engaging than solitary workouts. There is also added musical interest as the original lines dance back and forth between the two parts. So grab your horn, a friend, and have fun!