Double-stops are where we play two strings often side by side, neighboring strings, and we play them up and down a scale in a melody, and it fills out the sound quite nicely. That's a little melodic fragment from an instrumental tune. Here I'm going to do the same thing using the third and second string together as double-stops. There's the first and second string. And you can take that up and down the neck, out of different positions, different keys. You can take different pairs of strings, get the same effect.
When you're working on these double-stops, again use melodies that you have down cold even if they're a kid's song from your childhood. Try to use a melody that you already know really well. You're not learning the melody. You're learning the technique. Okay? So have fun with that and keep picking.