I cited Toni Arthur because she has a trained voice, very nice if you like art song but it always struck me as sounding out of place in the duo with Dave Arthur.

I'm trying hard to imagine Our Shirl doing madrigals but can't. Has she ever?

And I don't know what point you are attempting to make re: Maddy & Sandy. Is it just that they have both fronted a folk-rock band? Shirley has done that too, remember. As has June Tabor yet she is rather heavily into art song these days (discounting her latest CD, obviously).