I was just looking for the same thing and picked up these books: One is Martha Meier, Jazz, Rags, Blues Duets for two. Some of them have some more complicated rhythms (dotted eighths, syncopation) but but some are pretty simple. Also, a book by Carolyn Miller, Just Play it! Together: Six early elementary level piano duets, and Terrific Tunes for Two, Martha Neier: Exciting Duets for Elementary to Late Elementary Pianists. I also found Martha Mier's Santa Fe Sunset, which is early intermediate.
Pizno