The story behind these duets, passed down in the 1808 book Biographical Sketch of Michael Haydn by Georg Schinn and Joseph Otter, both students of Michael Haydn, goes like this:

“Mozart, who visited him daily, heard of this, sat down and wrote for his afflicted friend with such indefatigable industry that the duets were finished in a few days, and were given [to the patron] under Haydn’s name. In later times we have often delighted ourselves with these marvelous works of charity, the original of which our master preserved as a sacred relic.”

This scenario provided an interesting and rewarding opportunity for Mozart, too. It just so happened that Michael Haydn’s “patron” was none other than the Archbishop of Salzburg, the former employer and “old tormentor” of Mozart. Since the archbishop was unaware of Mozart’s surrogate role, one can only imagine that writing these duos must have been a source of great amusement for Mozart. His glee is particularly reflected in the “roguish theme” in the last movement of the Bb Duo, as it was likely a private joke at the expense of the archbishop.

The archbishop never learned the true authorship of the final two duets, which were not published during Haydn’s or Mozart’s lifetime.

So how did Mozart get the music to look and sound like Haydn wrote it?

Well, Mozart didn’t sign or date the duets’ autographs, most likely to help conceal his authorship. And since writing for this instrumentation was a fairly new venture for him, he might have taken fewer chances and been more concerned with disguising his writings as Haydn’s than with his usual brilliant use of counterpoint.

In any case, Mozart’s wit and cunning have once again proven his mastery in the art of composition, and the true nature of his friendship with Michael Haydn.

5 tips for playing the duets
1. On the first chord in the opening movement of the G major (K. 423), practice breaking the quadruple-stop into two lower and two higher notes, and then try to connect them.
2. Adjust your dynamics to reflect who has the melody and who has the accompaniment, and be ready to switch back and forth.
3. Stay in the lower half of your bow when playing the staccato 16th notes to find the best spot for a controlled bounce.
4. Imagine touching your finger on a hot stove when playing the quick grace notes to help get the reaction and response you need.
5. Listen to and study how both parts intertwine, trade off the melodies, and fit together. This will give you a better understanding of your individual part’s function, and how to play it.