Mozart’s Concerto for Two Pianos and his sophisticated “Linz” Symphony, composed in just four days, are two of the composer’s mature orchestral works. Program Nine, “Friends, Family, and Students,” presents them alongside examples by those within his inner circle: the Overture to Der Schulkandidat by his close friend Maria Theresia von Paradis, a musician who lost her sight at an early age; the festive Symphony in C by Marianna Martines, at whose musical soirées he was a frequent guest; the Overture in D by Franz Xaver Wolfgang Mozart, his youngest child, born just four months before his death, who went on to study with Salieri and Hummel; and the “Turkish” Symphony by Franz Xaver Süssmayr, the friend, student, and assistant of Mozart’s, who is best-remembered for his posthumous completion of the latter’s Requiem.