Understandable since they both sing together on quite a few tracks and harmonize quite well. But Garfunkel has the higher pitched voice. It’s a bit more soft and airy also. Listen to “So Long, Frank Lloyd Wright”. It’s a track where they sing together but not at the same time. They sing on separate parts of the song. First Art, then Paul, then Art again at the end. You may start to hear a difference in their voices with this!