Please see Double Stop Shop Parts 1 & 3 lessons #8572,#8621, and Baby Blues Part 3 lesson #8271, which applies some of these double-stop concepts.
In part 1 we looked at using 5th and min3rd interval double stops on the adjacent E and B strings. Now let's see how to play tunes with many changes using only a tritone (b5) interval.
Here is one tritone "shape" which can be used to play over any tune using only dom7 chords, such as the good ole' 3 chord blues.
How can this fingering be both chords? The notes are C#(Db) and G. C# is the third of the A7 chord, and G is the 7th. Db is the 7th of Eb7 and G is the 3rd.
On to page 2 with an example of how to use these.