Complex sentence: He was a student of Ben Silver and Sanford A. Moeller, renowned rudimentalist, who popularized the Moeller method, and within two years he was playing opposite Krupa at the 1939 World's Fair in New York.
Split into simpler sentences: He was a student of Sanford A. Moeller, a rudimentalist who popularized the Moeller method. Within two years Chapin was playing opposite Krupa at the 1939 World's Fair in New York.