It’s hard enough for parents trying to raise twins, but how about a theater attempting it? Well, those crazy show-tune-loving folks at Signature Theatre have taken on this intimidating, exhausting task with a pair of fraternal musicals being raised side by side on their Shirlington stage.

After a visit with both of them on a single weekend day, the first at the matinee and second in the evening, one received an education in why such projects are virtually unheard of. Few stage ventures are more daunting than creating a musical. And as the company reveals in its dual world premieres of “The Boy Detective Fails” and “The Hollow,” absolutely nothing is more difficult than two of them.

Neither of these solidly sung pieces comes across as anything close to a finished work. I’m glad, for instance, that I had read Joe Meno’s fine novel “The Boy Detective Fails” before I saw the musical adaptation on which he has collaborated with composer-lyricist Adam Gwon, because otherwise I don’t think I would have been able to glean the rationale for this narratively challenged and emotionally impoverished show.

The experiential hollowness of “The Hollow,” by book writer Hunter Foster and composer-lyricist Matt Conner, and based on Washington Irving’s celebrated short story “The Legend of Sleepy Hollow,” has less to do with a certain opaqueness than a decided flatness. To put it bluntly, it’s a snooze. Neither spooky nor comically infused, the musical unfolds as a somber evening service of ballads and chorales. Some songs, like the plaintive “Goodnight Prayer,” delivered silkily by actress Whitney Bashor, possess a supple beauty. But they coalesce around no urgent dramatic idea.

While there can be delight in the mere process of exploring new musical works, a company’s rolling out of two problematic premieres at once forces a reviewer to make unpleasant choices. So if I have to choose, I would say that “The Boy Detective Fails” offers the more intriguing potential. Meno’s modernist, pulp-ish novel illuminates the despair of Billy Argo, a sensitive, anxiety-ridden soul who gains renown as a precocious crime solver in his fictitious home town of Gotham, N.J. He’s convulsed by many outside stimuli, the most tragic being the suicide of his beloved sister, Caroline.

The whimsy of Meno’s literary achievement might transfer to the stage more successfully if the work were more widely known. As it is, the detachment of the central character, played by Stephen Gregory Smith, makes Billy a musical-theater cipher. The most vital thing about a dramatic character cannot be simply that he was the author’s impetus for a book.

Meno and Gwon strip away some of the novel’s other characters and subplots and try to develop a story around the haunting of Billy by his sister’s death. There are encounters in a halfway house with Billy’s villainous nemesis, Professor Von Golum (Thomas Adrian Simpson, looking a lot like Christopher Lloyd in the “Back to the Future” movies). They sing a couple of lively duets together, in a rather unremarkable score. But “Boy Detective” never makes clear who the heck this professor fellow is, or what, for that matter, Billy and he are to each other.

The most promising interlude evolves between Smith’s Billy and his quirky love interest, a kleptomaniac named Penny, who’s played with a tiny wisp of Tina Fey by the talented Anika Larsen, who sings the sweet ballad “Little Mysteries.” In most other respects, “The Boy Detective,” as directed by Joe Calarco and simply adorned by set designer Derek McLane and costume designer Kathleen Geldard, is a puzzle awaiting its own more resonant solution.

“The Hollow” somehow manages to shirk off its central mystery entirely. How can a musical about a menacing evil just beyond the village of Tarrytown, N.Y., fail to raise a single goose bump? The musical, directed by Matthew Gardiner, deals too decorously with the bland, surreptitious courtship of Bashor’s Katrina Van Tassel and the handsome outsider Ichabod Crane (Sam Ludwig), doomed like so many from this disturbing enclave to disappear in the night.

Conner found far more useful material in the poetry of Edgar Allan Poe, which he adapted as lyrics in his intense and sensual mood-musical, “Nevermore,” at Signature back in 2006. This time he, in collaboration with Foster, eschews suspense and ends up with a work that replaces chills with mere coldness.

The Boy Detective Fails

Music and lyrics by Adam Gwon, book by Joe Meno, based on his novel of the same title. Directed by Joe Calarco. Orchestrations, Andy Einhorn; choreography, Karma Camp.

The Hollow

Music and lyrics by Matt Conner, book by Hunter Foster. Directed by Matthew Gardiner. Orchestrations, Michael Morris.

Both productions: Sets, Derek McLane; costumes, Kathleen Geldard; lighting, Chris Lee, sound, Matt Rowe; music direction, Gabriel Mangiante. With Evan Casey, Sherri L. Edelen, Tracy Lynn Olivera, Margo Seibert, Harry A. Winter, James Gardiner. Through Oct. 16 at Signature Theatre, 4200 Campbell Ave., Arlington. Visit or call 703-573-7328.