Works by JS Bach are a great way to build chops and dexterity while playing some great music at the same time. Bach wrote a series of two-part inventions for harpsichord which work well as two-guitar duets. Although this is a classical piece, you can use a pick if you want to. I find that a pure alternate-picking approach is fine. The melody part is spread out over pages 1-3 while the companion bass part is in the 2nd sequence. In each page, you can listen to just the tabbed guitar, or both parts at the same time.