Falling between a single note and a triad, a double stop is an interval – a diatonic third, for example – and can be played on adjacent or non-adjacent strings. While we often associate guitar solos with single-note leads, playing two notes at the same time can serve as a useful approach to both solo improvisation and rhythm guitar parts.
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Guitarists like Chuck Berry, Stevie Ray Vaughan, Steve Cropper, Danny Gatton, and Isaiah Sharkey all utilize double stops in their playing.
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(Link) For more, refer to Isaiah Sharkey’s double stops lesson in his Pickup Master Class.