If two concert pianists are performing a work written for two grand pianos, there are two ways you can position the instruments. They can sit side by side, an arrangement known as ‘twin beds’. Or they can be slotted together so the performers face each other. That’s called a ‘69’.

When Martha Argerich and Stephen Kovacevich play together, they opt for twin beds. Appropriately, you might think, since they’re divorced — but really it’s because Kovacevich insists on sitting so low that Argerich can’t see his head if she’s opposite him. With everyone else she prefers a 69, as do most pianists: it’s easier to make eye contact.

And that’s crucial, because it’s no joke trying to synchronise with a duo partner when your own part is monstrously difficult. No wonder the two-piano repertoire is neglected.

Indeed, it might have disappeared by now if Martha Argerich hadn’t decided, more than 30 years ago, that she was too nervous to play solo in public. (Ironic, when you consider that she possesses the world’s most impregnable technique; if only similar jitters would silence one or two overrated celebrity pianists.)

Instead, she enticed musicians of the calibre of Kovacevich, Nelson Freire and Daniel Barenboim into joining her in arrangements of Rachmaninov’s Symphonic Dances, Stravinsky’s Rite of Spring and Ravel’s La valse — plus the handful of masterpieces originally written for the medium, such as Mozart’s Sonata for Two Pianos in D major K 448, Brahms’s St Anthony Variations and Rachmaninov’s Second Suite.

Some of the most rewarding two-piano works are reductions of orchestral scores — though perhaps ‘reduction’ is the wrong word. Losing the orchestration can give you more of the music. The Rite of Spring is every bit as barbarous in the hands of Argerich and Barenboim as it is under the baton of Boulez; it’s just that the dissonances are spelled out differently. And if you want to make sense of Strauss’s Sinfonia Domestica, first try Argerich and Alexandre Rabinovitch in Otto Singer’s transcription, which strips away the special effects to uncover a forceful argument. Then hear what Furtwängler does with it.

None of this is to be confused with music for four hands on one piano. That has its own repertoire, dominated by Schubert, and its own magic: it can be enchanting to hear two voices singing from the same keyboard. However, lots of pianists aren’t crazy about the physical contortions or yielding the sustaining pedal to the other player. Also, they worry about personal hygiene. ‘I’m drenched in sweat,’ one of them told me after playing Schubert’s F minor Fantasy. ‘And it’s not my sweat.’

But back to two-piano music, because it lies at the heart of the London Piano Festival, now in its second year at Kings Place. I’d rather miss a season of Proms than these concerts. They include solo recitals by pianists who, in a world that valued musicianship above showing off, would be household names: for example, Danny Driver and Lisa Smirnova. There are major-league soloists, too — this year, Nelson Goerner, last year Kovacevich — and on the Saturday they all leap into a two-piano marathon curated by the festival’s artistic directors, Charles Owen and Katya Apekisheva.

Owen and Apekisheva are a world-class piano duo to rank with Yaara Tal and Andreas Groethuysen and the late Duo Crommelynck (‘late’ because Patrick and Taeko Crommelynck committed suicide on the same day in 1994).

Unlike those duos, however, you couldn’t say that they’re more than the sum of their parts. Owen and Apekisheva are ferociously gifted soloists, as we heard in the first half of Thursday’s opening concert. Owen gave us Brahms’s Two Rhapsodies Op. 79 with a bouncy spontaneity that revealed how much the music owes to Schumann; then a sequence of Liszt pieces ending in an exquisitely voiced Liszt-Wagner Liebestod. Apekisheva played the Second Piano Sonata of Mieczyslaw Weinberg (1919–96). Weinberg’s echoes of Shostakovich verge on pastiche, but she brought out something fragile and elusive — a touch of Poulenc, perhaps?

One thing Owen and Apekisheva really know how to do is make a piano sing, even when the score bites and lurches. In Rachmaninov’s Second Suite — a piece that makes me question the conventional wisdom that Tchaikovsky was a greater composer — their Steinways sang to each other, melodies lifted and rhythms sharpened by the finest chamber acoustic in London. The applause was foot-stomping, a tribute not just to the performances but also to the special thrill of music for two pianos. Let’s hear more of it, please.