I assume you're looking for 2 parts without organ? 2 parts with organ is pretty easy. There are a number of Renaissance bicinia collections (Rhaw's Bicinia gallica, latina, germanica 1545.the Lassus duos, etc.). These tend to be a little problematic as service music as they are often chunks from Masses or other partial texts (and not so often from the Propers either). There's tons of medieval stuff, if you want to go there.

Jenny, great site. Thank you.