McGuinn did not cook up this idea on his own. The idea of doubling just a string or two has been around for a while. Spider John Koerner primarily used a 7 string guitar with a doubled G - String on the records he did with Dave Ray and Tony Glover. He credited some older blues person with the idea.

I expect Art Thieme to jump in on this thread. I think it was a 9-string he retrofitted to get more oomph from his guitar. Again, he got the idea from an older bluesman.

The names of the bluesmen escape me. I expect someone will come up with the answers.

Roger in Baltimore