From its earliest days, cinema has attempted to show viewers things that aren’t really there. Often, though, movies that withhold visual information are more powerful than those that reveal it all. And as Brothers of the Head and The Night Listener demonstrate, even films that don’t display everything can still show too much.

Directors Keith Fulton and Louis Pepe know that leaving some gaps in the narrative can boost a film’s sense of wonderment. That’s why, in Brothers of the Head, they use a now-familiar tactic to defy traditional fiction’s omniscience: They pitch their story, the tale of two literally symbiotic ’70s rockers, as a historical documentary that can only reveal what a previous set of filmmakers managed to capture on celluloid. Stylistically, this gambit largely works, in part because Fulton and Pepe know the genre from the inside: They’ve made several nonfiction movies, notably Lost in La Mancha, the 2002 account of Terry Gilliam’s failed attempt to film Don Quixote. Yet the picture’s successful aspects aren’t all a matter of form. They also include a compelling cast and something that most fictional pop-music flicks can’t muster: a persuasive score.

Adapted from a 1977 Brian Aldiss novella, Brothers of the Head is the tale of two sensual young men who are a lot closer than, say, the gangster and the rocker who swap lives in Nicolas Roeg and Donald Cammell’s epochal 1970 freakout, Performance. Tom and Barry Howe (Harry and Luke Treadaway) are conjoined twins, sharing internal organs and linked by a flap of skin. They should have been separated as infants, says a surgeon, but instead their father—mistrustful of doctors—exiled them to a remote cottage on a section of the Norfolk coast that just happens to be called “the Head.” At 18, their environment dramatically changes, when Dad contracts his boys to Zak Bedderwick (Howard Attfield), a vaudeville veteran turned manager who grasps the freak-show pedigree of fop pop and glam rock. He locks the Howes in a stately manor, where a group of unempathetic rock-’n’-roll tutors trains them as the frontmen of a combo dubbed the Bang Bang.

Tom, the stronger and nicer one, learns how to play guitar and writes pleasant ditties like “My Friend,” a tribute to his new paramour, journalist Laura Ashworth (Tania Emery). Barry, the charismatically weaker and meaner one, responds with proto-punk blasts like “My Friend (You C***).” An introductory number, “Two Way Romeo,” speeds the band to Britpop-style overnight fame, and two filmmakers arrive: American documentarian Eddie Pasqua (Tom Bower) and hysterical British romantic Ken Russell (playing himself). Their unfinished movies, supposedly, provide the raw footage for Fulton and Pepe’s retrospective film portrait.

The title of Aldiss’ book also refers to a third head that the brothers share; in the movie, this unsettling detail has been downgraded to an internal growth that doesn’t spoil the Howes’ pouty appeal. Fulton and Pepe present the twins’ conjoinment as more an emotional issue than a physical one, and scenes that purport to show the brothers’ corporeal link merely undermine the illusion that the actors are actually conjoined. (The fleshy connection doesn’t, for example, hamper Tom’s guitar playing.) If the mechanics of the Siamese-twin relationship always seem a little dubious, the plot is briskly propelled by its understanding of Britpop frenzy, as well as by Clive Langer’s songs. A member of Deaf School during the period simulated by Brothers of the Head, Langer is best-known for co-producing the likes of Madness, Elvis Costello, and Morrissey. The tunes he’s written (or occasionally co-written) for the Howe brothers are a shade too extreme for 1975 but are otherwise as exemplary as the Treadaway brothers’ rock-star bearing.

Yet if the attitude, music, and period details all click, that’s still not enough. Brothers of the Head doesn’t recall the eyeliner-flaunting variety of ’70s British rock just in sound and feel, but also in theme. Identity is slippery, image is powerful, fame is corrosive, and those who trifle too much with any of these forces will be destroyed—just as in such ’70s ego-smashing trips as Performance and The Rise and Fall of Ziggy Stardust and the Spiders From Mars. While Fulton and Pepe pose as historical filmmakers, they show no evidence of benefiting from three decades of hindsight. Rather than question glam rock’s adolescent self-analysis, they simply second it, yielding a film that’s less analysis than sham artifact.