Two of the world’s leading string quartets, the Ebène and the Belcea, come together to perform octets written 75 years apart by two phenomenally gifted teenage composers: Felix Mendelssohn and George Enescu. Of the concert of the Mendelssohn and Enescu octets in Philadelphia in November 2024, The Strad said: “The phrase ‘luxury casting’ gets tossed around a lot, but seemed …