According to a popular anecdote it was Michael Haydn, Joseph's younger brother, who was responsible for the composition of Mozart's two Duos for violin and viola. In the summer 1783, two years after being released from service to Archbishop Colloredo of Salzburg, Mozart returned home to introduce his family to his new wife, Constanza. He called on Michael Haydn but found him in a terrible state, sick in bed and extremely worried. Haydn had been commissioned by the aforementioned Archbishop to compose a set of six duos for violin and viola. He had completed four but was too ill to finish the other two. The Archbishop was withholding payment pending their completion, and Haydn was in terrible need of money. Mozart saw the dual opportunity of doing his friend a good turn while trying his hand at a new format. Two days later Mozart returned with the two duos, K. 423 and 424. All that was needed was Michael Haydn's signature on the manuscript and a messenger to take them to the Archbishop. There can be no disputing the historical importance of Michael Haydn's venture into the composition of duos for violin and viola. Numerous composers have since composed works for the combination and have produced masterworks far exceeding the simpler works begun by Haydn. Mozart's duos are relatively unknown to the concert going public yet are great favorites of violinists and violists who love to play them as lighter entertainments.
-- Kendall Briggs