Please see Double Stop Shop Parts 1 & 2 lessons #8572,#8605, and Baby Blues Part 3 lesson #8271, which applies some of these double-stop concepts.
Did my description get your attention? Sound confusing? Or mayble like I don't know a thing about music? Well, read on, and I'll explain.
You may recognize the shape as an Ami7 chord, but we are only going to play two notes, E and C. Think of it as a C chord with the 3rd (E) in the bass.
I present it once as a chord, and once as a sequence so you can hear what the notes of this interval shape sounds like together and separately. This interval, or the distance between the notes, is a 6th, minor 6th in this case. So you see, this minor 6th interval has the root and 3rd of the chord I have in mind.
Here's another double-stop, this time a major 6th.
I chose to call it a Dmi chord cuz' it has the third, F, and the root, D of that chord. Remember, we're learning how to use double-stops to comp or play over chord changes, so we need to associate these shapes and sounds with chords.
Onto page 2, where we will extend these double-stops throughout the whole C major scale.