Watch the Open String Triple Stops: 8 online guitar lesson by Robbie Calvo from SoloCraft
Take the time to analyze the different shapes over the G and C chords and start to determine for yourselves why each shape works. The clue is - sweet notes - if the notes are in the chord it will work and resolve - sweet notes! Try adding some effects to your sound like reverb or delay like I do, it will help the notes sustain and ring out more. A little overdrive might sound cool too. This is our last jam session together, make the most of it. Mix up both ideas I have shown you and get creative.