In his book “Collaborative Circles: Friendship Dynamics and Creative Work,” from 2001, the sociologist Michael P. Farrell made a study of close creative groups—the French Impressionists, Sigmund Freud and his contemporaries. “Most of the fragile insights that laid the foundation of a new vision emerged not when the whole group was together, and not when members worked alone, but when they collaborated and responded to one another in pairs,” he wrote. It took Monet and Renoir, working side by side in the summer of 1869, to develop the style that became Impressionism; during the six-year collaboration that gave rise to Cubism, Pablo Picasso and Georges Braque would often sign only the backs of their canvases, to obscure which of them had completed each painting. (“A canvas was not finished until both of us felt it was,” Picasso later recalled.) In “Powers of Two: Finding the Essence of Innovation in Creative Pairs,” the writer Joshua Wolf Shenk quotes from a 1971 interview in which John Lennon explained that either he or Paul McCartney would “write the good bit, the part that was easy, like ‘I read the news today’ or whatever it was.” One of them would get stuck until the other arrived—then, Lennon said, “I would sing half, and he would be inspired to write the next bit and vice versa.” Everyone falls into creative ruts, but two people rarely do so at the same time.