On paper, the Civil Wars shouldn't work at all. Joy Williams is 29, brought up in California and grew up on blues and country. John Paul White is 10 years her senior, from Alabama in the deep south, and spent years in failing rock bands. Together, however, the singer-guitarist duo have shifted hundreds of thousands of albums and picked up two Grammys, and have such an uncanny chemistry that they have been mistaken for a married couple.

That chemistry is musical, not sexual, based around their entwining voices and sublime harmonies. When Williams reaches the high notes and White provides the bottom end, it is like several decades of American music – from sugary AOR back to the Byrds and especially the Everly Brothers – have been combined into one accessible package that is easy to adore.

At what White describes as their biggest-ever ticketed gig, the audience handclaps start as early as the third song, From the Valley –just one of more than a dozen terrific songs played with heart and care. For a duo with one single instrument (Williams adopts a keyboard to accompany White's guitar for just one number), they make a big sound; and when things threaten to become too syrupy, White suggests the crowd "get more rowdy" as they pile into the raucous Led Zeppelin/White Stripes-style stomp of Barton Hollow, with its darker lyrics about death and the devil. Later, on Oh Henry, the sweet-piped Williams unveils a surprising inner rock chick.

The comical banter between the elfin, chatty, cutely dressed Williams and the lanky, straight-faced, besuited White is part of the show. "I sing this from a female perspective," he says, prompting her to instantly retort: "That's not such a big stretch." Their standing ovation leaves Williams thoroughly overwhelmed, while White pretends to play it cool: "That's more like it!"

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