Millie and Christine McKoy, conjoined twins and entertainers, were born into slavery on a plantation in Columbus County, North Carolina, in the southeastern part of the state. Their parents, Jacob and Monemia, were slaves owned by Jabez McKay, a blacksmith; the twins later adopted McKoy, a version of their master's surname, as their own. Their father was of pure African descent; their mother was both African and Native American. The twins were conjoined at the lower spine and stood at an approximately 90-degree angle to each other; they shared a single pelvis, though each had a full set of limbs. At birth the frail Millie, who weighed an estimated five pounds, seemed to be only an attachment to her twelve-pound sister's back, and henceforth the two were always known as a single "girl" named Millie-Christine, whom other family members referred to as "Sister." As adults they reached a combined weight of 170 pounds.

Conjoined twins were not unheard of at the time of Millie and Christine's birth. Indeed a famous pair, Chang and Eng, lived on adjacent plantations in northwestern North Carolina with their wives and children and numerous slaves. Chang and Eng, born in Siam, had been brought to the United States several decades earlier by the American impresario P. T. Barnum. Billed as "the Siamese Twins," the pair had appeared in his traveling shows for many years before retiring to North Carolina in about 1840. The name coined by Barnum was eventually used to describe all conjoined twins, though it was not applied to the duo known as Millie-Christine during their lifetime.

McKay, owner of the twins, quickly recognized their commercial value. When they were ten months old, he sold them for $1,000 to an agent for public exhibition. Over the next few years the twins changed hands several times, their price increasing to perhaps as much as $40,000, before settling with Joseph Pearson Smith and his family. Millie and Christine were hired out by Smith to various road shows, where they were billed as "the Carolina Twins." For several years they toured the South and later other parts of the United States, appearing as sideshow attractions along with jugglers and acrobats as well as other "freaks," as human "oddities" were then called. General admission was twenty-five cents; visitors had to pay an additional fifty cents to view the girls.

By 1854 Smith had lost track of the now three-year-old twins, who were then appearing at Barnum's American Museum in New York City, billed as "the celebrated African United Twins." The following year, after appearances in Canada, the twins were taken to England under the aegis of two other contemporary showmen, "Professor" W. J. L. Millar and William Thompson. For more than a year they toured cities throughout the British Isles, captivating audiences and creating a sensation wherever they appeared. To counteract public skepticism, the twins were usually subjected to physical examinations wherever they appeared, a practice that ended only in their early teens, when the twins refused to be subjected to further intimate scrutiny.

By late 1856 Smith had traced the twins to England, and early the following year he traveled to London, accompanied by the twins' mother, Monemia, to reclaim them. Smith had earlier purchased both parents and the twins' numerous siblings from McKay, and they were now living with the Smith family. After a lengthy trial, Smith regained legal custody of Millie and Christine and returned with them and their mother to the family plantation in North Carolina. While other McKoy family members worked as slaves for the family, the twins were groomed for careers as stage entertainers. By all accounts the twins were precocious and highly intelligent, and Smith's wife taught them to read, write, and present recitations in German and French as well as English; she also taught them to sing and dance and to play the piano. Soon the twins were making public appearances to great acclaim throughout the country as well as abroad, under Smith's personal management. Billed variously as the "Two-Headed Girl" and the "Two-Headed Nightingale," the duo known as Millie-Christine presented acts that included musical performances and declamations of verse that they had written themselves.

When the Emancipation Proclamation brought an end to slavery in 1863, the twins along with their parents and siblings chose to remain with the Smiths. After several years Jacob McKoy bought a nearby farm with money donated by the twins, who were now earning some $600 a week. With Joseph P. Smith, Jr., acting as the twins' manager after his father's death, their show business career continued at home and abroad for nearly three decades, and they were among the most celebrated entertainment attractions of their day. The well known and the obscure alike clamored to see and hear them sing, dance, play the piano, and recite poetry, and the twins encouraged their popularity by writing an autobiography, History and Medical Description of the Two-Headed Girl, that was widely circulated following its publication in 1869. Among their most famous fans was Queen Victoria, who invited them to Buckingham Palace for several performances in the summer of 1871 and presented the pair with diamond hairclips.

In the early 1880s the twins were a featured attraction with P. T. Barnum's traveling circus, and they continued to make public appearances at fairs and carnivals until the end of the decade, when Millie's failing health forced the pair to retire to their home in rural Columbus County. There they continued the charitable work they had begun years earlier, supporting African American schools and churches in the South. By the end of the century the twins had long faded from the public eye, and they lived the rest of their lives in obscurity. Early in the twentieth century Millie was diagnosed with tuberculosis, and the twins spent time at an unidentified sanitarium in the Northeast in an unsuccessful effort to arrest the disease.

Over the years various inquiries had been made about the possibility of separating the twins, and though this was declared impossible while both remained alive, there was some discussion of performing surgery after the death of one of them, in hopes of saving the life of the other. As Millie's condition worsened, a local doctor consulted with physicians at Johns Hopkins Medical College about postmortem surgery but was advised not to do so; indeed the Johns Hopkins physicians recommended that Christine be given massive doses of morphine to end her life quickly after Millie's death. This course was apparently pursued, following a petition to the state governor for approval, though Christine, who was in good health, outlived her sister by a number of hours, possibly as many as seventeen. They never married.

For biographical information, see the twins' 1869 autobiography and Joanne Martel, Millie-Christine: Fearfully and Wonderfully Made (2000), which includes illustrations and a bibliography. In addition see the anonymously written Biographical Sketch of Millie Christine, the Carolina Twin, Surnamed the Two-Headed Nightingale and the Eighth Wonder of the World (n.d.; ca. 1902-1912); the full text, which includes photographs and other illustrations, is available at http://docsouth.unc.edu/neh/carolinatwin/carolinatwin.html. For a popular account of Chang and Eng, see Irving Wallace and Amy Wallace, The Two (1979). Lucid discussions of conjoined twins are in Luigi Gedda's pioneering study, Twins in History and Science (1961); see also Francis R. Packard, History of Medicine in the United States, vol. 2 (1931). For discussions of conjoined twins as carnival acts, see Robert Bogdan, Freak Shows: Presenting Human Oddities for Amusement and Profit (1988); Frederick Drimmer, Very Special People (1973); and Rosemarie Garland Thomson, ed., Freakery: Cultural Spectacles of the Extraordinary Body (1996). See also Bruce Feller, Under the Big Top (1995); and Tom Ogden, Two Hundred Years of the American Circus (1993). In addition see Leslie Fiedler, Freaks: Myths and Images of the Secret Self (1978). A local newspaper account of the twins' death is in the Whiteville (N.C.) News Reporter, 13 Oct. 1912.