Lots of information about Marais and Miranda here. Click on "Biography" at the top for even more.
I think they delved pretty deeply into the folklore of many countries, but they were definitely concert performers rather than "folkies." A lot folk music enthusiasts nowadays may find them a bit "tepid" for their taste. Nevertheless, at a time when the only folk singers the general public knew anything about were Burl Ives and Susan Reed (in the Forties, before the Weavers' first songs hit radios and jukeboxes), Marais and Miranda were also fairly well-known.
John, classic guitarists don't like barre chords any better than anybody else, but if that's the only way to play the notes they want to play, they bite the bullet and do them. Whereas Richard Dyer-Bennet occasionally played a classic solo or two in concert (I saw him do a very nice job on Fernando Sor's Etude in Bm—lots of barre chords), he never recorded any. Josef Marais used a classic guitar, and although I don't recall him ever playing any classic guitar solos either in concert or on records, he did use classic guitar technique in his accompaniments, and he seemed like a pretty competent guitarist. He once collaborated with a fellow named Adrian Van de Horst to write a basic manual for beginning classic guitarists. I've still got it around somewhere.