This dual fretboard in silver and gold represents how the same theme has been realized famously in two distinct versions that both "sound so splendid" (or, as the original lyrics say, "Das klinget so herrlich").
In this month's sightreading handout*, the first exercise is the original 1791 theme well known to opera fans, and the second exercise is a simplified version of a famous 1821 piece for classical guitar that claimed to be the same theme despite some differences. Its four variations have been acclaimed as "a testing ground for every aspiring guitarist". Enough hints. Do you recognize either of these versions from the score?
Play both exercises in first position, then follow the story and youtube links on the back page to appreciate how they're related. Whichever is your favorite, you'll "make new friends and keep the old --- one is silver and the other gold".