At this oddly elevenish age of 77

somehow, I seem to've chosen the Fantasia of JS Bach's Keyboard Partita in A minor for my Signature Piece. I've got so I can play all of its different segments perfectly in three or four sittings. Why indeed?  The Main Thing about this Fantasy is its absolute and utter Twoness. Play the left hand by itself, it makes perfect musical sense. Play the right hand by itself, it also makes sense.  But when they are played together, they also make sense, and, as has been noted, it's a different sense.