I've stumbled upon this amazing footage of the great Ramón Montoya playing guitar with his nephew Carlos Montoya. It's a movie from 1938, but in all these years I don't think anyone in flamenco ever noticed. I checked the complete cast of the film, and it identifies them by name, so there's no question.

This is the link...they're accompanying Imperio Argentina.

https://www.youtube.com/watch?v=swXjWjsIuhQ

Although they're in the background, interesting bits can be gleaned, most notably the fact that one is playing por medio and the other por arriba. I wonder if this was when young Carlos decided it would be more fun to be a soloist...