A year ago this month, SOPHIE closed out PC Music’s first official showcase at SXSW. No photos could be officially taken of the headlining set, and SOPHIE could hardly be seen. White floodlights backlit the English producer as a crowd of sweaty young people danced. Most of us had waited hours to stand across a stage from SOPHIE, and the silhouette added to the mystery and ambiguity that has helped fuel SOPHIE's appeal.

At last night’s Hype Hotel showcase in Austin, SOPHIE walked onstage like SOPHIE was eager to be seen. After playing one song, “MSMSMSM,” alone behind a spread of electronics, the producer was joined by Charli XCX for the remainder of their first-ever co-headlined performance. Together, they played the entirety of Vroom Vroom, the SOPHIE-produced EP that Charli released last month, interspersed with singles from each of their discographies – their individual songs knitting together to form their work. SOPHIE's pitched-up vocal samples squealed under Charli’s live singing on reworked versions of her hits. Her “Doing It” melted down from a bright radio mainstay to a dark industrial menace, clanking along on SOPHIE's artificial synth work.

Neither artist is a stranger to collaboration — SOPHIE worked on "Bitch I’m Madonna," and in some quarters, Charli is best known for her "Fancy" cameo – but neither has so totally plugged into another artist's work until now. When Charli lends her accented vocals to other artists’ tracks, she appears as a guest, differentiated by her own stylistic idiosyncrasies. When SOPHIE produces alongside Diplo on “Bitch I’m Madonna,” a mile-wide formal gulf separates the two. Together, Charli and SOPHIE feel like the same gloriously glam, PVC-clad monster, but with two heads; it’s clear they originate from the same primordial goo.

Charli swung her long ponytail to the whip-crack synths of “Paradise,” then lip-synched and bounced to BDSM banger “Hard.” She drank something yellow out of a plastic cup when SOPHIE churned up the opening bubbles to “Lemonade,” and SOPHIE sang along in bright red lipstick to “Boom Clap."

Few pop pairings seem this symbiotic. Charli softens SOPHIE's shell, while SOPHIE sharpens Charli’s songs to a riskier edge: Instead of radio's bubblegum politeness, the singer gets to indulge kink, aggression, sexual energy. Radio Charli feels sanitized by comparison, while studio SOPHIE now sounds almost timidly detached. Together, in person, the duo challenge each other, egg each other on to moments of real danger, and grow from the messy, feminine space where they meet. It’s thrilling to watch a relationship like that form, and it's even more thrilling to anticipate where it will lead.