In the second of two programmes of works set for this year's GCE A-level exams, CHRISTOPHER HOGWOOD looks at some mixed doubles: Tippett's Concerto for two orchestras, Bartok's Sonata for two pianos and percussion (also in its orchestral version) and Brahms's Quintet in F minor for piano and strings (also in its two-piano version). Record requests from the under-20s.