Frustrated and feeling isolated in Salzburg, Mozart (now age 21) set out in 1777 with his mother to Mannheim, Paris, and Munich in the hope of finding for himself a more significant court position. It was a devastatingly unhappy trip. Not only was he turned down at all three courts, but his mother died unexpectedly while they were in Paris. Disappointed and still grieving, he returned to Salzburg early in 1779, resigning himself at least for the time being to his position there as court organist. Soon thereafter, he wrote the Two-Piano Concerto, presumably for himself and his sister Nannerl.
Though there are certainly moments of darkness in all three movements, the piece overall is an expression of great joy, delight in the possibilities of pianistic virtuosity, and exuberance. As with so many other composers, sad times do not necessarily produce sad music!
The two solo parts are absolutely equal, sharing the dialogue between them in countless different ways, and placing the orchestra more as accompaniment than in Mozart’s concertos for solo piano.
The equality of the two solo parts surely demonstrates that Nannerl must have been every bit as virtuosic a pianist as her brother. Later in Vienna, Mozart played the piece several times with one of his students, Josepha Barbara Auernhammer. It was with her in mind, too, that he later wrote the Two-Piano Sonata in D, K. 448, perhaps as a companion piece to K. 365. When we consider the list of people for whom Mozart wrote piano concertos – Countess Antonia Lodron and her daughters Aloisia and Josepha (the Concerto for Three Pianos, K. 242); Louise Jenomy (K. 271); Barbara Ployer (K. 449 and 453); Maria Theresia Paradies (K. 456) – it is clear that both Salzburg and Vienna could boast of any number of first-rate women pianists.