Martha Mier has a duet book for beginner students. I'm sorry, I don't remember what it's called. They're very nice pieces, both parts are equally musical and are of equal difficulty.
Ravel - Pavane de la bell au bois dormant, from Ma mere l'oyeCzerny - Allegretto, Op. 824, No. 18Stravinsky - Five Easy Pieces: Andante or Balalaika are good ones.