Onstage there was a crackling romantic energy between them, although both were married to other people and always maintained that their lovelorn duets and ballads were pure artifice. Ms. Williams’s husband, Nate Yetton, managed the band and was literally in the wings. In July 2012 Ms. Williams gave birth to their first child, Miles, who traveled with the duo during its last tour.

Ms. Williams said that creative disagreements had been building for several months and came to a head in the songwriting sessions on her screened porch after her son was born. “I have a lot of ambition,” she said. “I believe in pushing borders and finding new territory and expanding. I personally felt like John Paul was more of the ilk that things would happen organically and ‘If it ain’t broke, don’t fix it.’ ”

Ms. Williams said becoming a new mother had nothing to do with the breakup. She also dismissed rumors that she had romantic feelings for Mr. White. “I was not in love with John Paul,” she said. “But I was, and am, in love with the band.”

Yet she acknowledged that the frantic pace of touring had also taken a toll on both marriages, and that last year their aesthetic differences had grown personal. Mr. Peacock, the producer, said the strains were evident when the duo went into the studio to record the album last September. Making the first album had been almost effortless — “like taking a Polaroid,” he said — but this time Ms. Williams and Mr. White seemed to be pulling in different directions. “The emphasis of their collaboration had shifted,” he said. “If they were on the side of John Paul’s ways of working before, Joy’s ways were now dominant. While John Paul is nothing if not a total pro, I could tell that too much analysis, talking and the creating of options is just not his sweet spot. Joy on the other hand, loves to turn over every stone in search of the best among a thousand choices.”

There were no shouting matches or ugly scenes, Mr. Peacock said, but the music seemed to reflect the underlying combat. Mr. White pushed to add a growly, sharp-edge electric guitar to a few tracks, while Ms. Williams “led the charge toward the darker, grittier sounds.”

While the tension increased, “the chemistry remained,” Ms. Williams said. Saying it would be “unladylike,” she declined to say what event or words precipitated the split, which came after a performance at the Roundhouse in London on Nov. 6. “It wasn’t one thing,” she said. “It was a lot of little things. Each of us wanting something separate.”