I tried out a Stagi Hayden-system duet at the Button Box last week, and it made sense to me, much more so than the McCanns I've tried.
And Robin, with a little finger flexing it's possible to chord on the C/G Anglo in keys like D, F, Dm, Cm even A. Can't do chords with melody too well in those outer keys, but there's enough there for accompanying singing.