Oh, if you're talking 2 pianos there is MASSES of great pieces... some favourites:
Mozart, 2pno sonatas, adagio & fugue c minor, concerto for 2pno & orchestra in Eb (no.10)
Brahms, sonata f minor op.34b (or is it 34a?)
Brahms, Haydn variations
Rachmaninov, suites
Poulenc, sonata, concerto 2pno & orch., and a sweet little piece called "l'embarquement pour Cythere" (excuse my french!)
Debussy, En blanche et noir
Stravinsky, "Concerto per due pianoforti soli"
Infante, Danses Andalouses
Milhaud, Scaramouche
Lutoslawski, Paganini variations (the best 2pno piece??)
Messiaen, Visions de l'Amen
And many many many more...
and if we're talking 1pno, four hands...
Mozart, sonatas again, and fantasy in f minor
Schubert, fantasy in f minor (a very popular key when 4 hands are involved it seems!)
Poulenc, sonata
And, again, much more than that...
Anyone got anymore suggestions?? Anyone played Schumann Andante & Variations for 2pno, 2vlc & french horn?? There is also a shorter version for 2pnos.