DOUBLE-STOPS COME FROM ARPEGGIOSWhen I was first learning to play mandolin, I learned a few songs by slowing down the music to half-speed (at that time, it was done by transferring the song onto a old reel-to-reel tape machine my dad kept in his den). However, I hit a snag when I wanted to learn a song where the mandolinist was playing two notes at once, commonly called "double-stops." It wasn't until I learned a little bit of music theory that the secret to this became clear: Double-stops are just two notes from the arpeggio played at the same time.
Say you play a C major arpeggio (the 1st, 3rd, and 5th notes of the scale). Pick any two that you can play at the same time, and it will be a C double-stop. You can use a C double-stop anytime the band is playing a C chord. Here's a tab example.