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 b5 is the upper voice. The sound produced with this technique will remind you of B3 organ style playing.
We'll begin with the b5 of the C blues scale 6th string root form in the seventh fret of the 2nd string. This will be the upper voice. Then you'll play the C blues scale that exists below it on strings 3 through 6.
You can also use the b5 which occurs in the eleventh fret of the 3rd string as the upper voice.
The b5 is rather sinister sounding, ao apply these licks judiciously.