Complex sentence: Rachmaninoff played the solo part in his Second Piano Concerto, and Vaughan Williams, at Wood's request, composed a short choral work for the occasion: the "Serenade to Music" for orchestra and 16 soloists.
Split into simpler sentences: Rachmaninoff played the solo part in his Second Piano Concerto, and Vaughan Williams, at Wood's request, composed a short choral work for the occasion. The work he wrote was the "Serenade to Music" for orchestra and 16 soloists.