If you're into anti-war songs, the Leon Rosselson song 'ACROSS THE HILLS' is a good one as it's a dialogue between two voices.

The traditional "WHISTLE, DAUGHTER, WHISTLE" can be done as a dialogue, with one voice as the mother and one as the daughter.

There's the old chestnut " SCARBOROUGH FAIR" as well - the Simon & Garfunkel version with the canticle part.

These are just three suggestions that I can think of off the top of my head, but I know there are a lot more - I just haven't trawled through the books yet! There are loads of non-dialogue songs that work well in two- or three-part harmony as well - I'm one-third of an all-female band myself and most of our repertoire is a capella arrangements of traditional folk songs, so I know there are lots that work that way!

Sophie