As you already know this cover of Tennessee Waltz is made up of two parts. The first part, in fingerstyle, is in the key of G major as are the majority of interpretations of this song. I particularly like Eva Cassidy's version, and it was the inspiration for the harmony in my cover.
The second part, played by trumpet and guitar, modulates to Bb major. This part is based on Doc's Severinsen version. The trumpet here plays a pretty straightforward melody while I follow one octave higher with the second voice on guitar. This simplified harmony allows some room for improvised phrases that I introduce here and there with the guitar. I hope it inspires you to come up with your own improvisations!