“It’s the kind of thing where I don’t need to think about the outfits that much anymore,” she added.  Egos abound on both sides of the celebrity styling business as the architects of A-list facades have gained followings to rival their clients (see: Jen Atkin, Kardashian tress tamer). Stylists have publicists. Tag teams are rare. But joining forces in 2007 allowed Mr. Zangardi and Ms. Haenn, both former stylists for MTV’s on-air personalities, to play up their different strengths and sensibilities. Ms. Haenn veers toward hip-hop (her first styling job was on the set of the video for Trick Daddy’s “Take It to da House”). Mr. Zangardi is more pop: “Mandy, Britney, Christina Aguilera, the boy bands,” he said the other day over coffee with Ms. Haenn at the Soho House here in West Hollywood. Ms. Haenn grew up in Brooklyn, Mr. Zangardi in Ohio.  “Mariel is the bad cop,” Mr. Zangardi said.  Ms. Haenn said: “Look, I’m comfortable being like, ‘No, that’s not my favorite,’ or, ‘No, I don’t like that,’ even if they do. They get used to getting that from me and getting what Rob brings.”  “It’s Ohio versus New York,” Mr. Zangardi said.  Their first joint effort was styling Rihanna’s 2007 “Umbrella” music video, which featured computer-generated water splashes and a series of good-girl-gone-bad ensembles (a leather romper, a low cut baby-doll dress). More work for the singer followed: a horned headpiece from Rihanna’s “Rockstar 101” video sits on a shelf in the duo’s studio here, though Rihanna is no longer a client. In 2009, Andrea Lieberman, a stylist whom Ms. Haenn looked up to, moved away from longtime clients like Ms. Stefani and Ms. Lopez to start her own clothing line, A.L.C. Ms. Lieberman said she was relieved to see her star clients end up working with a strong team.  “It was important to me that when I left both women, they were in supremely good hands,” Ms. Lieberman said about Ms. Stefani and Ms. Lopez. “I knew Rob and Mariel had it. They seamlessly could edit the best of the world of fashion with the world of music and had an incredible work ethic. It was a no-brainer for me to suggest them both to Jennifer and Gwen.”  Ms. Lopez recalled her first meeting with Ms. Haenn and Mr. Zangardi, in 2011. “I found them to be shy and kind of quiet, but they had their point of view,” she said. “We started working together, little by little, and it worked really well. Those things you can’t really force, you have a sensibility or you don’t.”  Mr. Zangardi and Ms. Haenn have since scripted many head-turning moments for Ms. Lopez: the mustard colored, cape-capped Giambattista Valli gown she wore to the Golden Globes in January, the crystal-studded Dsquared2 bodysuit she opened the American Music Awards with in 2015. The bodysuit was displayed in a glass case, along with Ms. Lopez’s other sartorial hits (that green Versace dress), outside a recent “All I Have” performance in Las Vegas. Mr. Zangardi was there to ensure that the array of outfits Ms. Lopez would wear that night were ready for the spotlight. “She’s sliding on the floor in boots that are rhinestone that we have to recrystal every day,” he said. (Ms. Haenn was in Japan with Ms. Stefani.)  They work out of a windowless, fluorescent-lit studio, jazzing up dancers’ mall-bought outfits with safety pins and sequins (the budget for the backup crew is not what it is for marquee performers). In the back, a dozen bins of Spanx are stacked next to racks of old costumes worn by Ms. Lopez and Shakira, a former client. Despite the modest digs, clients like Ciara, Cara Delevingne, Rachel McAdams and Emily Ratajkowski often drop by to try on clothes.  At a recent fitting for the backup dancers on Ms. Stefani’s tour, Ms. Haenn, Mr. Zangardi and their in-house tailor, Anna Kenaraki (poached, with blessings, from Ms. Lopez), bopped along to reggaeton and debated the merits of side versus back zippers (back zippers, Ms. Haenn said, are less likely to get stuck during a frantic midconcert costume change). They discussed the sartorial effects of “bum flaps,” rectangles of fabric that frame the hips of Ms. Stefani and her dancers and create the illusion of movement even when they’re not doing much of it. These are conversations Mr. Zangardi and Ms. Haenn like to have.  “When you’re on a red carpet, you’re just doing this,” Ms. Haenn said, strutting two steps and squatting primly. “With what we do, we need to make sure you’re not going to get stabbed with a huge safety pin while you’re running around on stage.”
Rob Zangardi and Mariel Haenn build on the sensibility of their star clients to create onstage wardrobes and red-carpet moments.