Watch the G Bluegrass: 2 online guitar lesson by Carl Verheyen from S.W.A.T. Sessions: Country
Many double stops make an appearance here. Some of it's a carryover from my swampy blues material I taught in my first TrueFire course, SWAT Blues.
Whenever I'm playing single note scales it's always Mixolidian, which is the major scale with a flat 7th.