Watch the Double Stops online guitar lesson by Brad Carlton from Guitar Lab: Big Blues & Beyond
This lesson covers double-stop licks in which the root is the upper voice. The sound produced with this technique will remind you of B3 organ style playing.
We'll begin with the root of the C blues scale 6th string root form in the eighth fret of the 1st string. This will be the upper voice. Then you'll play the C blues scale that exists below it on strings 2 through 4.
This same approach can be used in the lower octave with the root occuring in the tenth fret of the 4th string. You then combine this root with the C blues scale below it.