Watch the How to Find Double Stops online guitar lesson by Robbie Calvo from Double Stop Chops
Double stops can be found within your chords, arpeggios, and scales, so I'll be showing how to notice them and how to create them from each of those elements. It should be noted that chords and arpeggios are all derived from your scales anyway, so another way to look at all of this is to simply say double stops are derived from scales in the same way that chord harmony is constructed. Once we dig into the nuts and bolts, you'll start to see the relationship between the harmonized major scale: The chords and double-stops follow the same principle of maj7 - min7 - min7 - maj7 - dom7 - min7 - min7(b5). The shapes will always be the same unless the string pairs use the B string in the unit, because the B string is tuned a 3rd above the D, not a fourth like all the other string pairs. This means you'll have to get used to making those adjustments or learn the shapes that fall on that B string.
Let's take a look at that on the fretboard for a moment so that you can get a visual of what I am saying. The most important aspect to remember is locating the start of the scale regardless of string pairs that you'll use for your lines, that way you'll understand that the shapes adhere to the harmonized major scale and its chord intervals, too.