Joachim Raff's '12 Pieces for Piano 4 Hands' offers a collection of accessible and melodically rich works for two players. The pieces are characterized by their pleasing harmonies and often evoke a sense of gentle sentimentality.
Developing sensitivity to ensemble playing and a shared sense of phrasing is key.
12 Pieces for Piano Four Hands (Op.82) is rated Henle level 4 of 9 — intermediate repertoire, roughly ABRSM grades 5–6. In the RCM system it corresponds to about level 5.
You should be comfortable at Henle level 4 — meaning you can already play level 3–4 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
A typical performance of 12 Pieces for Piano Four Hands (Op.82) lasts about 30 minutes.
12 Pieces for Piano Four Hands (Op.82) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.