Definition: In the dance music world, B2B or b2b indicates that two DJs will be performing onstage at the same time. This is indicated on lineups as “[DJ 1] b2b [DJ 2].” B2b is shorthand for “back-to-back,” so for example Feed Me b2b Kill the Noise can be read as “Feed Me back to back with Kill the Noise.” Sometimes b2b sets will be listed on lineups as “vs.” or “versus” – e.g., “Feed Me vs. Kill the Noise.”
Back to back does not mean that one DJ will be playing immediately after the other, even though in general English you would say that two events happening one after another, like baseball games, are back-to-back.
In EDM, the phrase back-to-back comes from the days when DJs played vinyl records. While one DJ would be managing the turntables, the other would be searching through their catalog of vinyl records behind the decks for the next record to play. This DJ would usually have his back to the audience, so the two performers would spend much of the show with their backs to each other. Nowadays, very few DJs, at least in large settings, spin vinyl records or even CDs. The rise of laptop-DJs and digital turntables has enabled DJs playing b2b sets to face the audience while they queue up the next track. (Unless you’re Above & Beyond caught in the rain at Ultra… but that’s a story for another day.)