Jennyr - Like Jim, I find it hard to imagine singing any of the ballads in my repertoire as duets because the sense of commitment/involvement is such a personal thing that no two people will necessarily be moved in the same way - what Sheila Stewart calls the "conyeach".
I regularly do ballad workshops with Gordeanna McCulloch and we share the same background of our introduction to the music, as well as most of the same influences or models. We offer some less familiar ballads, so that participants come with no preconceptions, and we all move through the learning process together. But the real point I want to make is that when it comes to individual verses, or sections, of the ballad concerned we regularly have to agree to differ - what sounds convincing to me might grate with her, or vice versa. Really what we are trying to do is persuade those attending that they have to find their own way of dealing with or interpreting tricky bits.
So I'm just a bit wary that any ballad duet would perhaps involve too many compromises to be a satisfactory experience for the performers as individuals. If the vocal pairing is a new one, why not work on more obvious songs for harmony treatment etc. and use the occasional ballad/narrative song as a solo for more variety?