Beth Lassi and Nina Zanetti have done a series of four lovely duet books. The Dulcimer Association of Albany has played some of them at meetings I have attended. I recommend them highly. Check them out at http://www.ninazanetti.com/. Enjoy!!
I include two duets (Morning Has Broken and Scarborough Fair, and a round, Oh How Lovely is the Evening, in my instruction book, Beyond the Basics . Details and sample tab at www.cabinhillmusic.com.