“First The Barber of Seville, then The Marriage of Figaro. Then what? … On Friday the Whitsun edition of the Salzburg Festival” – under Riccardo Muti’s baton – “revives an opera that just might serve as the missing third panel: I Due Figaro (‘The Two Figaros’), by Saverio Mercadante.” (The second Figaro is none other than Cherubino, all grown up.)