Sorry for any possible confusion! A double stop is any two notes (hence "double") played simultaneously. The two notes of a double stop can be any given interval apart.

In the video, it is specifically an interval of a 6th between the two notes of the double stops being played. This is a very common blues-rock technique. Sometimes it is accomplished by hybrid picking because you have to avoid the B string. Other times, players simply mute the B string while striking all 3 top strings.

Hope that helps for clarity! :)