(Photo credit: Monika Rittershaus) Who’s afraid of double bills? We all love “Cav.” and “Pag.,” and there is some room in everyone’s heart for a “The Human Voice” meets “Bluebeard’s Castle;” nevertheless, Gluck’s Iphigenia operas performed together was a bit of a challenge. The operas, lasting two hours each, are easily stand-alone pieces, and—even though they share a protagonist—they appear {…}