What makes a collaboration work? In the case of the unexpected and seemingly unlikely one between solo acts Aimee Mann and Ted Leo, more factors are in their favor than you might think.

When the duo - who headline Union Transfer as the Both on Saturday - toured together in 2012, "I started watching Ted's shows," says Mann, talking on the phone over a shared line with Leo from Brooklyn.

"There was a new song of his that I really wanted to play bass on," she says. That song was "The Gambler," the first track on their new album, The Both. "So I asked to sit in. It's really interesting to hear the sound he gets with just one guitar. I also thought he was really funny. I thought, 'I'd love to be on stage with that.' It's great to have somebody to joke around with and share the vocals with and sing the harmonies with."

Word of Mann and Leo's teaming surprised the indie world, as Mann is seen as a contemplative singer-songwriter, and Leo a raucous rocker with punk roots.

"There's much more overlap than that broad brush of differences," Leo chimes in. "We both tend to have a considered approach to our lyrics. We genuinely want to say something. And our love for melodies and harmonies, I think, also overlaps. At the same time, Aimee operates at a level with those things that's like a higher tide that raises my ship."

"Ted puts across a certain attitude that I feel like I'm writing in my songs but I can't really express because my voice can't do that," Mann says. "Energetically, it doesn't come with that kind of force. So one of the fun things is to hear parts that I'm writing in my head in a way that I think I'm singing them, then I listen back, and it's like, 'Oh, I've turned this into a little folk song again.' I like the more rock elements I'm able to put in there with Ted."

The pair started writing songs together early in 2013, with initial plans to put out a six-song EP. Instead, they wound up with an 11-song set, whose single, "Milwaukee," features a video shot at the Boot & Saddle in Philadelphia. "Recording the first batch of songs just felt so good," says Leo. "Why wouldn't we keep going?"

The timing was right. "There was an aspect of our solo careers where you feel like you're treading water," says Mann. "I guess for me, there's a little of: 'Nobody buys records, so, really, what's the point?' Which can translate into, 'Nobody cares.' "

"Writing music is a compulsion. We'd probably do it no matter what," says Leo. "But in terms of continuing with this model, making records and touring, you do get these existential crises a couple of times a year, when you're just writing alone."

"You need to feel that somebody cares somewhere," says Mann. "And when you're working with another person, you do. It's, 'We care.' "

Both songwriters have lived on the road. Mann's band 'Til Tuesday scored its hit "Voices Carry" in 1985; Leo, 43, played in a succession of punk bands in the early 1990s. "The love of playing and performing doesn't decrease," says Leo, "but the grind of the toil takes its toll."

Has the Both renewed that love of performing? "It did for me," Leo says.

"It's a bit of a 'What the [heck],' " says Mann, 53. "Everything is a complete gamble. It's all going to the house, anyway. We might as well do what's purely fun for us."

The Both is an ongoing project. Mann and Leo have talked about writing a musical. And though they're musically compatible, that's only half of it.

"Having a good back-and-forth, the same kind of standards, and a similar harmonic vocabulary, there's all that," Mann says. "But so much of it is not being a giant baby when someone doesn't like your ideas. Or admitting that your song is not working or could be better.

"Honestly," she says with a laugh, "so much of it amounts to not being a baby about stuff. We're able to give each other a break and kind of laugh and move on."

"I know there are people who are interested in another solo record from me," Leo says. "But the world is not exactly beating down my door. So while Aimee and I have this thing going on and it is so fulfilling, I feel like that's where I need to put my energy."

Mann puts it more succinctly: "Go where the love is."