Watch the R&B Tonality: Major Flavor online guitar lesson by Robbie Calvo from Double Stop Chops
I'm going to just reiterate some nice 4ths, 6ths, and 3rd double stop ideas against this A7 bluesy R&B vamp. Basically, we're taking double stops from the key of D and applying them over the V7 chord from that key, so that means you're looking to resolve ideas from the key of D towards the V7 chord. In other words, A Mixolydian. Don't let those terms freak you out - you'll be able to hear what works and of course I'll show you where the "sweet notes" are, too!