“Dance these days — spring ’59 — is decidedly split into two main factions,” wrote the dancer Paul Taylor: neoclassical ballet, a modernist update of classical ballet, and modern dance, which broke free of ballet’s strictures to use movement as an expressive tool. These two genres were epitomized by the work of the choreographers George Balanchine and Martha Graham, respectively. “So when it’s announced that the two giants will collaborate...