Right now I am playing Beethoven's Sonata op. 49 no. 1, and Chopin's Duex Polonaise op. 26, no. 1. My duet partner is currently playing Chopin's Prelude in C, and Fur Elise.
In general we both like fairly fast pieces, but melodic ones.
I have played the first movement of Mozart's Sonata for 4 hands in D.