Co-headliner tours are like communism: They only work in theory. Sure, two acts of allegedly equal fame will draw a bigger crowd, but everyone involved—concertgoers, industry folks, even the musicians themselves—know who the main attraction really is. In the case of Josh Rouse and Leona Naess, Rouse wound up with the short end of the talent stick.

The show at Philadelphia’s North Star Bar was running an hour behind schedule when Naess appeared and calmly took the stage dressed in a T-shirt and jeans. Sipping a cocktail of tea and whiskey from a paper cup, Naess explained she was happy to be drinking again; she’d just quit taking antibiotics to fight a lingering cold. Naess has a fascinating background: the daughter of a shipping tycoon, the stepdaughter of Diana Ross and the former fianceé of Ryan Adams. She just released her third (and best) record—a stripped-down, self-titled album about wide-eyed heartbreak produced by Ethan Johns (Rufus Wainwright, Ryan Adams, Jayhawks).

Backed by a drummer, pianist, bassist and cellist, Naess strapped on her guitar and began playing two of her older songs (“Mexico” and “Charm Attack”) before gracing the audience with beautiful renditions of her new material. During a heartbreaking part-a capella, part-piano version of “Ballerina,” she pulled up her hair to reveal a pained expression. Her naturally hoarse voice—rougher than usual due to the extensive touring—occasionally gave out, which only added to the emotional effect of her songs.

Between tunes, Naess joked how the people on the venue’s balcony could only see the top of her head. Naess danced, she played and she made the live interpretations of her songs feel frighteningly real. Lyrics such as “Nothing is as sad as a man with bells on his shoes,” (“Don’t Use My Broken Heart”) show her to be a more intense and mature songwriter on Leona Naess. The passionate expressions on her face and the clenching of her fists showed the audience what they didn’t hear on her previous albums: These songs are deeply personal.

The crowd cheered for an encore and shouted requests. “Now I feel like I’m going to disappoint everyone,” said Naess, who only had time for one more song. The finale of recent single “Calling” was well-received.

Although Naess and Rouse have publicly expressed fondness for one another’s music, co-headlining is still a tricky business. Rouse takes the stage last with every show they play—it’s part of the deal. The better part of the crowd that cheered Leona was there for the more-popular Rouse, anticipating the songs they already know by heart. He stood before the audience and stared into space, looking far more disengaged than his cheerful bandmates.

Rouse had the potential to put on a great show, yet somehow managed to be unimpressive. He hardly moved and talked only to hear his own voice, introducing songs and sharing pointless stories (“Love Vibration,” for instance, was written in Australia. Yeah, that’s the whole story). He played songs from each of his five albums but appeared bored with his own show. Rouse live—even on more energetic songs like “Love Vibration” and “Marvin Gaye”—sounded the same as listening to the albums on high volume at home. Not even a cover of Steely Dan’s “Dirty Work” could boost the level of enthusiasm.

The crowd was trying to get into it. They bobbed, they sang, they drank—but the truth is that Josh Rouse is a cocky, mediocre live performer. He loves the attention, but doesn’t love his songs. Only a revamped attitude could have saved him. Lesson to learn: If you’re going to co-headline a tour and announce on your Web site that you are touring with “special guest Leona Naess,” either make sure she sounds horrible or perform like the great musician you have the potential to be. Otherwise, go back to the safety of an opening act and hope your audience doesn’t tire of you.