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, Tom and Barry Howe (Harry and Luke Treadaway), conjoined twins. They should have been separated as infants, says a surgeon, but instead their father exiled them to a remote cottage on a section of the Norfolk coast. 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 un-empathetic 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. Their unfinished movies, supposedly, provide the raw footage for Fulton and Pepe’s retrospective film portrait.

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. 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. Langer is best known for co-producing the likes of Madness, Elvis Costello, and Morrissey.

When the attitude, music, and period details all click, that’s still not enough. 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.

BROTHERS OF THE HEAD ( * * * 1/2 )

Directed by Keith Fulton and Louis Pepe. • Starring Harry Treadaway, Luke Treadaway and Tania Emery. • R, 90 min. • At the Osio Cinemas.