Watch the Double Stops Ideas online guitar lesson by Brad Carlton from Guitar Lab: Big Blues & Beyond
Lesson 2 explained double-stops as applied to the minor pentatonic scale. The pattern that was revealed was the "high" and "low" sides of the scale. You played two adjacent strings, and always moved each note in the same direction.
This lesson changes the initial interval, but still moves through the scale. The pattern is always every other string out of the high and low sides of the scale.