“MANET/DEGAS,” the fall blockbuster at the Metropolitan Museum of Art, New York, begins with an unabashed, double-barreled bang: Édouard Manet’s last great self-portrait, paired up alongside one of Edgar Degas’s first. The juxtaposition provides a thrilling object lesson in the stolid compare-and-contrast curatorial methodology that defines the exhibition, but if it’s meant to show the two artists on an equal footing, it doesn’t stage a fair fight.