“We work together like a pair of connected muscles that contract or expand, depending on what the other one is doing,” says Christian Gerhaher—who completed his doctorate in medicine before becoming a singer—about his collaboration with Gerold Huber. Such intuitive understanding seems par for the course for two musicians who have been performing together at the highest artistic level since their teenage years. Now the acclaimed duo returns to the Pierre Boulez Saal with a program of songs by Franz Schubert.