Sister, sister

“TWOFERS,” they are called, otherwise known as Siamese, or conjoined, twins: genetic mutations caused by their parents’ or grandparents’ exposure to fallout from the “Penitence Project,” a series of radioactive explosions generated by the U.S. government in the Nevada desert to punish its citizens for Hiroshima and Nagasaki. Thousands, perhaps millions, of these fused twins now walk the earth -- only “backward” countries engage in the brutal process of surgical separation -- enough to warrant a full-fledged civil rights movement (“Togetherism”), a political lobby and an intellectual society (“Siamists”). Subjected like any minority to discrimination, out of their suffering comes a pride parade, a film festival and lobbyists who fight for specially designed airplane seats that accommodate two heads on a single body. San Francisco, of course, leads the country in legislating equal rights, which include the right to marriage, a union many people believe should exist only between “singleton” men and women.

This is the premise of Shelley Jackson’s first novel, “Half Life,” and it’s ambitious, even for a writer whose most recent effort involves tattooing an entire short story, one word at a time, on the bodies of more than 2,000 volunteers. It is neither quite science fiction nor dystopian fantasy. The story never claims, for example, that humans have suffered because of radiation; instead, its twofer progeny represent a unique challenge to individuated humans to rise above their prejudices and make room for a new breed. Here, Jackson taunts plausibility, toys with common assumptions and comments boldly on the conundrum of human existence: To what extent do we desire to be alone? This is a delicate feat, balanced uneasily between parable and parody, and for the first 100 pages, at least, it’s natural to wonder whether the writer can pull it off. She almost does.

Jackson presents “Half Life” as the first-person narrative of Nora, who since birth has shared most of her body with her sister Blanche. “Dicephalus dipus dibrachius,” explains Nora. “That’s two heads, two legs, and two arms: standard-issue twofer.” (In this world, there are no mutants joined awkwardly at the tops of their skulls; the two-headed, single-bodied twofers function normally in every imaginable way.) From the start, Nora, the “verso” twin -- on the left -- has been the more dominant of the pair, given to experiments that test her “recto” twin’s nerve. “If we stood on the double line of the skinny highway twenty feet past a blind curve,” she wonders, “while a semi going fifty plunged down a thirty-percent grade toward us, would Blanche jump right or left?” At other times, Nora attempts to manipulate her sister’s thoughts, persuading her that her perceptions fall short of reality. (We call that “gaslighting.”)

But sometime during the twins’ childhood, the unexpected happens: Blanche falls asleep. At the start of the book, she has been unconscious for 15 years, blithely unaware of Nora’s struggle to feed and shelter the body they share by working for a phone sex service. We get to know Blanche only through Nora’s reveries about their lives as children: How they learned to sit quietly and read separate books; how they hid from sight in a public pool by diving deep and exploring all the lost things in the cracks. How one day, they went for a walk, a knife balanced between their heads. “The edge was just touching the skin of her neck,” Nora recalls, “right where tan gave way to pallor under her hair. A fine muscle in her neck was standing out. I could sever it easily.”

Nora does not decapitate her sister that day, but the notion resurfaces as Blanche begins to stir, commandeering their shared limbs from the depths of her decade-and-a-half slumber to send various objects hurtling through the air. “Lithobolia,” Nora calls her, after a character in a story their grandmother told about a creature who throws things. She makes lists of the damage: “Three crutches duct-taped together”; “Acupuncture model of cat”; “Cat.” Blanche plays other tricks too, inserting entries in her sister’s journal, groping the breasts of an acquaintance in public. Tortured and humiliated, Nora looks for ways to rid herself of her twin.

Jackson, who wrote the 1995 hypertext novel “Patchwork Girl,” has said in interviews that her work is concerned -- maybe even obsessed -- with the body. In addition to her tattoo project, “Skin,” she is the author of a collection of stories, “The Melancholy of Anatomy,” which includes tales of cancerous tumors big enough to crawl into, a menstruating city and a culture enchanted by phlegm. Her sensibilities, which play out in “Half Life’s” morbid descriptions of Nora and Blanche’s beloved road-kill collection -- they dress up dead wild things -- as well as vivid decapitation fantasies and hallucinations of neighbors sitting down to a snake dinner, are at once enticing and sickening, like an exhibit at Culver City’s Museum of Jurassic Technology.

Jackson’s lurid fascination with the corporeal also emerges in the substance of Nora-Blanche, their beating hearts not quite in sync, their overlong third collarbone giving way to separate heads, Blanche’s sleepy spittle and whistling breath, their fraught and premature birth, the way they were rejected at first by their mother and fed gruel by a sensitive father. The author has so mastered the art of description that her characters rise from the page soft and sticky; at times they even smell.

But “Half Life” is not concerned with body lust for its own sake. While Jackson devotes much of the book to Nora’s quest for “unity,” she also explores the tangle of all human intimacy, the blending of selves in friendship or marriage, the odd configurations of people that function as families. There is Nora and Blanche’s forward-thinking clan, made up of their mother and father, plus Dad’s mom, Granny, and Mom’s girlfriend, Max (“we were taught we were extraordinary,” Nora says). There is Nora’s friend and roommate, Audrey, who wins Nora’s affection when she wonders whether Blanche ever “checks in” and will thus need her own shelf in the kitchen cupboard. While Nora battles against her conjoined fate, Audrey, a twofer wannabe who makes experimental films about the twins she envies, tries to persuade her friend to attend seminars on “Venn theory,” a strategy by which twofers can make peace with their togetherness. One Venn-aligned couple, in fact, has found a way to blend two separate personalities -- one Rastafarian and one upper-class Brit -- on a single body, and in the intersection of selves they create singular music.

And yet, it’s near this point that “Half Life,” for the most part sure and true to its own weird logic, begins to wobble, not by extending the metaphor of twinness too far but in connecting it too obviously to real-world concerns. Because in real life, author Leslea Newman wrote a controversial children’s book called “Heather Has Two Mommies,” “Half Life” contains a reference to a book called “Heather Has Two Heads.” (This makes no sense, since among the twofers, each head has its own name.) Because the pope has been summoned to weigh in on assisted suicide and terminated pregnancies, “Half Life” features Catholic priests asking the Vatican “to make a formal statement declaring conjoined twins to be two individuals possessing equal right to life.”

These excessively literal parallels, entertaining in the first part of the book, become wearying toward the end, just as Jackson’s filigreed prose, with its adjectives turned into verbs (a slope “gentles,” “darkness weltered” ), loses its seductive power after a while and comes out forced. “Half Life” is a humane and heartfelt book nonetheless, often goofy, always ingenious and sometimes magical. But with a more ruthless editor’s knife, this rough-edged story might have been a polished gem.*