I’ve been listening to this Robert Johnson CD lately, and it sounds like he’s playing two guitars simultaneously. What’s this technique called? More importantly, how is it done?
Am I correct in assuming this is very difficult to do?
You’re linking to an Amazon listing for the Complete Recordings. Which track are you specifically thinking about?
He played fingerstyle - no flatpick and setting up an alternating bass line with his thumb while doing melody on single notes or partial chords. Yes, it is hard, but once you find the groove it is really fun. I can do it a bit, but am no expert…
This what you are looking for? 
Robert Johnson was a Mississippi blues singer and songwriter, who according to legend, sold his soul to Satan “at the crossroads” in exchange for his remarkable talent on the guitar.
http://st-james.hubpages.com/hub/Robert-Johnson-the-Legend--the-Devil--the-Crossroads--and-27
Not if you’re a good negotiator, in the right time and place…
I’ve wondered, if you’re not going to be actually using your soul for a while, could you just lease it to the devil for, say, a couple of months?
And to further illustrate the technique:
(Listen for the high piccolo part at about 1:50 into it)
I’m sure there are better videos but Hubert Dublin, who played guitar for the howling’ wolf for decades, it think played in a similar style. The above link has Hubert tearing it up with Clapton, jimmy Vaughn, and Robert Cray. Real guitarist feel free to correct me on Hubert’s style.
I won’t correct you on his style, but I will correct you on his name: it’s Hubert Sumlin! (Auto-correct, maybe?)
General consensus is that Robert Johnson went down to the Crossroads and sold his soul to the devil for the ability to play guitar with such virtuosity. That doesn’t make it true, but it is the story.
The story about selling his soul to the devil at the crossroads was originally told about Tommy Johnson. The legend somehow was transferred to Robert Johnson, possibly because he’s better-known to the general public.
Six fingers on the left hand, perhaps.
Per Elijah Wald, Escaping the Delta: Robert Johnson and the Invention of the Blues, the Faustian motif was a common one among Johnson’s contemporaries, but it was more prevalent for dice players than guitarists. It’s just one of those things that got attached to Johnson more firmly because he didn’t live to get much mainstream media exposure - he’s more a creature of mystery than someone like Son House or Muddy Waters who did several interviews explaining their technique.
Here’s a video of Stefan Grossman playing Mississippi Blues - give you a sense of the technique:
Different players had very different styles, approaches to melody and rhythm, etc.
And yeah, This American Life has a great piece on uncovering the whole Crossroads mythology and how it migrated to Robert Johnson…
Keith Richards’ recent autobiography talks a good deal about difficult licks and techniques (including some he says he was never able to figure out or master - boggle) and I think he talks about the difficulty of emulating Robert Johnson.
Then again, since KR has clearly sold his soul as well, he may not be the best authority. 
Tommy Johnson was older and more famous at the time. I think Robert sometimes claimed he was Tommy’s brother, and that may have helped “transfer” the legend.
My guess is that Robert having a song called Crossroad Blues may have had something to do with it, too.
Yes… except there’s nothing in that song about selling his soul to the devil.
Doesn’t matter. The legend is specifically that he sold his soul at the crossroads.