Duets are fun and a great way to learn to play with others, a skill that can often be lost to pianists. Of course, any piece can be made into a duet - divide here, add a little there. I would love to do a recital with Whitewater Chopsticks made into a duet. Here is one arrangement. Although I would probably arrange it for four hands instead of two pianos.
Two years of "Piano Ensemble" in college consisted almost entirely of playing with the other pianists in duets and trios. Here are a few of the pieces I learned and watched in class.
Brahm's Hungarian Dance no. 5
Buy at: http://www.virtualsheetmusic.com/score/Hungarian1H.html
Free copy - The first ever published version - more difficult:
The Bercuese from Faure's Dolly Suite is a charming beginning duet for intermediate players, and is also available on Virtual Sheet Music. IMSLP or other printable classical score providers might have it for free.
Here is the Bercuese:
Also, listen to the world's most famous duet playing pianists, the Lebeque Sisters:
And for some real inspiration, listen to the Five Browns.