The Creek Dippers, The Borderline, London

Even if you knew nothing of their individual and linked histories, you would still smell something bittersweet in the Borderline air tonight. So to know exactly what brings Mark Olson and his wife Victoria Williams to this central London basement, at this moment in time, verges on the heartbreaking.

It's a story you can skip over if you have ever come across these singular singer/songwriters. Briefly, it goes like this: in the early 1990s, Olson was the lead singer of the Jayhawks, who were then – and still are without him – ever at home on the cusp of greatness. At the same time, Williams was an LA-based performer who had to be heard to be believed – think (if you dare) a cross between Tori Amos, Björk and Kate Bush.

Then, in 1993, Williams was diagnosed with multiple sclerosis. In a move unprecedented in rock, Olson quit the Jayhawks to look after Williams – with whom he had fallen in love and married – full-time. The pair moved to Joshua Tree, in the Nevada desert, built a small recording studio, and settled down to tend their garden, dogs and each other.

The rest, as they don't say, has been a deliberate escape from history. Which brings us back to the Borderline, one night on the 40-date European tour that has been hastily booked during one of Williams's remission periods. As she takes the stage, Olson helps her put her guitar over her head. But no one has to help her play it as the band (well, a violinist and a drummer) launch into some of the down-home songs from the couple's loose-limbed, free-form Original Harmony Ridge Creek Dippers records that have steadily trickled out of their home studio.

The songs are sweet and ramshackle and Olson – when he can take his eyes off his wife – is switching from bass to keyboard to guitar in a bid to hold things together. Williams, as ever, is away with the fairies. "I just had a thought during that song," she announces after "Frying Pan", from her 1987 debut Happy Come Home, "that we're are all on a spaceship," before adding, "Oh, had you all thought of that already?"

At another point, Olson straps the Fender on her and she looks at him puzzled, before asking: "Why have you put that guitar on me? What song did you want to play?" But Olson's answer is irrelevant. Vic has just swigged some water and, to celebrate, wants to play "Water to Drink", from her album of the same name.

The bittersweet shadow is cast by the fact that Olson – who let's remember was the leader of one of the tightest, most ass-kicking bands of the past 10-15 years – is little more than a bystander here. He looks tired, but he is clearly so in love with his kooky queen that tight, ass-kicking music has no place in his life right now.

When one cruel heckler persists with his request for "Waiting for the Sun", from the Jayhawks' seminal Hollywood Town Hall album, Olson's weary smile threatens to fade. And in the end, it is Williams who has to tell the crowd that "Mark probably doesn't want to play that song tonight." (As if she would, if only he'd let her.)

As Williams encores, solo, with "Moon River", you realise why the line "Two drifters, off to see the world" holds such allure for her. But the couple's positivity in the face of such a challenging life together, is best summed up by Williams's own lyrics from "Merry-Go-Round": "The key to the merry-go-round is the merry; the key to the fairy tale is the fairy; stuck on the bridge between happy and scary." That bridge is exactly where this incredible couple reside. It is why any live performance from them should be treated as the rare gift it is.