Thank you everyone. I really appreciate all this feedback. Arranging stuff could be good, as it is so difficult for beginners to play together - I especially like the idea of organising the players to play alternately a lot. I have already ordered some of the other stuff mentioned. Tunes For Two arrived yesterday, and I have to say is beyond them at the moment. But it will give them something to work towards.
I also found a pupil and teacher playing a duet on You tube, which they entitled Faster and Faster, and have simplified that (3 slow scales plus end riff for primo; single notes on minims - A F G E F D E A for Secondo) and taught it to them by rote this week. It is easy to learn and very effective, especially with me filling in with quaver arpeggios in the bass. A trio. Nice.
What a great resource this Forum is!