The cast, left, sing their parts from audience seats facing the musicians, right, during orchestral rehearsal at Signature Theater for their upcoming production of ‘The Boy Detective Fails.’ (Bill O'Leary/WASHINGTON POST)

It’s a Sunday afternoon rehearsal, and Signature Theatre’s small Ark space is snowed under with paper; it looks like the floor of the New York Stock Exchange after a nutty day. Actors and creative staff sit in the house, facing the six musicians on the stage. Everyone scrutinizes oversize, taped-together sheets of music as they put together the new musical “The Boy Detective Fails.”

Next door, in Signature’s slightly larger Max theater, two actors stand motionless on the wide, eerily empty stage. Above them loom dimly lit branches. The rest is darkness.

“They’ve been standing there for 25 minutes,” a staffer whispers. Somewhere in the murk, the director and designers are adjusting the lighting for the new musical “The Hollow.”

This week both musicals will premiere at the Max, in repertory — same designers, same orchestra, many of the same actors, and alternating on the same stage. Regional theaters seldom offer plays in repertory; premiering two new musicals in tandem is apparently unheard of.

It’s certainly a first for Signature, even with its tradition of taking on new material. And as the troupe hammered out contracts, they learned that they may be breaking new ground, period. The actors’ and musicians’ unions had no exact precedent for what Signature was trying to do.

The poster outside of the building illustrates Signature Theaters' ambitious staging of two new works at the same time. (Bill O'Leary/WASHINGTON POST)

“First of all,” says “Boy Detective” director Joe Calarco, “there’s nothing harder than the premiere of a new musical. Ever, ever, ever. And then to do two at once . . .

“I think I was probably drunk when I thought about it,” jokes Signature artistic director Eric Schaeffer. He’s sitting in the Marquis Theatre on Broadway; adding complexity to this rep are the facts that in July, Schaeffer moved his “Million Dollar Quartet” from Broadway to off-Broadway, and that now his production of “Follies,” which closed at the Kennedy Center in late June, is adjusting to New York (it’s in previews). So Schaeffer had no choice but to withdraw from directing “The Hollow,” and he isn’t around much to supervise or cheerlead. Matthew Gardiner, recently named Signature’s associate artistic director, stepped in.

But it was Schaeffer’s adoration of “The Hollow” and “The Boy Detective Fails” that led to the current ambitious pairing at Signature. In the summer of 2009, both pieces were workshopped together at the theater as part of the American Musical Voices Project: The Next Generation. “The Hollow” is an adaptation of Washington Irving’s “The Legend of Sleepy Hollow,” with music and lyrics by longtime Signature actor-composer Matt Conner. (His first musical for the troupe was “Nevermore,” based on the writings of Edgar Allan Poe.)

“The Boy Detective Fails” is based on Joe Meno’s novel about a Hardy Boys-style teen sleuth, now 30, who is deeply traumatized by his younger sister’s suicide. The Chicago-based Meno was e-mailed by up-and-coming New York composer-lyricist Adam Gwon about turning the show into a musical; nine months later they met face to face for the first time at the Signature workshop with the draft they had written long-distance.

The workshops concluded with public performances, and then Schaeffer summoned the writers to dinner. “I thought we were all going to be fired,” Conner says.

Instead, Schaeffer proposed the repertory idea.

Calarco says, “We were like, ‘Su-u-u-re.’ ” He’s sitting in the Signature lobby during a rehearsal break with Gwon, Meno and rep musical director Gabriel Mangiante, and Calarco’s skeptical inflection makes the group laugh.

But they’ve been hammering it together, so amid the paper blizzard last Sunday, the six-person orchestra wrangled with Gwon’s score. (“Our opening number, in band charts, is literally 99 pages,” Gwon says during the break.) The musicians listen attentively as orchestrator Andy Einhorn fine-tunes the arrangements and smooths out the latest changes.

Each show has its own orchestrator; Schaeffer felt that the musical styles were distinctive enough to warrant that. Conner says his music has a “classical sense, very structured,” and his lyrics even include a bit of Dutch to give a provincial quality to the Hudson Valley town. Gwon’s score, on the other hand, has the pep and adventure you’d expect of a kid detective tale; Meno admiringly says it has a “cartoonish” streak. Both shows feature what Mangiante calls “chiaroscuro” effects — light and dark tones in high contrast.

But guidelines from the musicians’ union suggested that the rep would have to employ a single orchestra. That, Mangiante says, became the subject of “big negotiations” as everyone looked for the combination of instruments that would do justice to each score, and for the six musicians with range enough to play them both. The percussionist stands amid a jumble of drums, xylophone, chimes and more. “He’s about 15 musicians by himself,” Mangiante says.

Mangiante is among the people who are straddling both shows, which Calarco calls “herculean” when Mangiante downplays the challenge. The fast-talking Mangiante does measure his words when he explains how he has managed: “It was just a matter of immersing myself in both worlds simultaneously. Except in order,” he says. “Does that make sense?”

He thinks the orchestra has it harder, coming to terms with two separate brand-new scores. “For them it’s just been a whole bunch of paper, and here we go!” Mangiante says. “Honestly, they look a little overwhelmed at this point.”

So much material, so little time: Gwon points out that “The Boy Detective” will go 10 full days without a run-through as both shows tag-team through technical rehearsals (the painstakingly slow piecing together of design and performance). Conner says something similar, noting that momentum in his “Hollow” rehearsals sometimes comes to an abrupt halt as his cast, on schedule, heads away to “Boy Detective.”

So surely there’s some jealousy along the way? A sense of having to give a little too much?

“To be quite honest,” Meno says, “I don’t feel like we’ve been asked to compromise anything.” Conner notes that he has a twin brother, so sharing is second nature to him.

Asked what he’s worried about, Schaeffer jokes again. “Selling tickets,” he says. New titles are always hard, and he’s doubled the problem. “Knock on wood, they’re both going to be great. But there’s a risk that one becomes a stepchild because it’s not received as well as the other one.”

And the lessons so far of trying two premiere musicals in rep?

“Anything can be done,” Calarco says, adding, “I would have asked for more rehearsal time.”

Says Mangiante, “I would have cloned myself.”

Pressley is a freelance writer.

The Boy Detective Fails

Music and lyrics by Adam Gwon, book by Joe Meno. Based on the novel by Joe Meno. Directed by Joe Calarco.
Starting Aug. 25.

The Hollow

Music and lyrics by Matt Conner, book by Hunter Foster. Based on “The Legend of Sleepy Hollow,” by Washington Irving. Directed by Matthew Gardiner.
Starting Aug. 23.

Both shows run in repertory through Oct. 16 at Signature Theatre, 4200 Campbell Ave., Arlington, 703-573-7328.