The Brahms *Sonata for Two Pianos, op. 34 bis" can be added to Dave's
list.  This is the earliest surviving (and some think best:-) version
of what is generally known as the "Piano Quintet".  The piece started out
life for strings with double cello and no piano, which Brahms destroyed
and re-cast for piano duo; it was at Clara's behest that he arranged it
for string quartet and piano. After it came out, Brahms wrote his
publisher extolling the virtues of the two-piano Sonata, urging them not
to forget it; almost like he arranged it for quintet under protest ...
but then again, I'm biased:-)

Leslie Kinton <[log in to unmask]>
Piano Faculty, The Glenn Gould Professional School,
Anagnoson and Kinton piano duo website: http://www.pianoduo.com