Set up.
Link up.
Hand over.

Two DJs in two places play one set. You each play your own DJ software on your own gear. DJ back 2 back sends your sound to the other DJ and plays theirs through your gear, so you can mix in. One of you is live at a time, and Handover passes it on.

First, download the app and sign up. Then set up your sound for your gear.

A laptop controller with rekordbox or Serato

1. Open your DJ software first, then DJ back 2 back.
2. In Your mix, pick your DJ software. The app sends only that app's sound, never your partner's, so nothing echoes back to them.
3. In Your partner, pick your controller: the output your DJ software uses. The app picks it for you when it can. Your controller plays your partner together with your DJ software; set how loud with Partner level.
4. To hear your partner in your headphones too, pick your headphone channels (3–4 on many controllers) in Headphones. If it isn't offered, see the Configure Speakers step.

rekordbox and Serato can't put outside sound on a deck, so you bring your partner up with Partner level, not a channel fader.

VirtualDJ: your partner on a deck

1. In Your mix, pick VirtualDJ.
2. Tick Into a VirtualDJ deck.
3. In Your partner, pick a virtual audio device that VirtualDJ can take as an input.
4. In VirtualDJ, set a deck's line input to that device. Your partner comes up on that deck like a track.

A club mixer (such as a Pioneer DJM) on channels 3–4

1. Connect the mixer to your Mac by USB.
2. Configure Speakers: open Audio MIDI Setup, pick the mixer, click Configure Speakers, choose a layout with 4 speakers, then Apply. Until you do, your Mac offers the app only channels 1–2.
3. In Your partner, pick the mixer, and in Channels, the pair that feeds the mixer channel you'll use (your mixer's manual says which). Set that channel's input to the computer (USB), and your partner is on its fader.
4. In Your mix, pick the mixer, and in Input channels, the pair that carries your own decks. Never the master: it has your partner in it, and they'd hear themselves come back.

The first time: macOS permissions

• Opening the app. It isn't signed by a developer yet. The first time, right-click it and choose Open, then Open. If there's no Open button, go to System Settings → Privacy & Security and click Open Anyway.
• System Audio Recording. The first time you connect with your DJ software as your mix, macOS asks to let DJ back 2 back record system audio. Click Allow; it only ever takes your DJ software's sound. If you said no, turn it on in System Settings → Privacy & Security → Screen & System Audio Recording. Without it the app hears silence, and tells you so.
• Microphone. Asked only when your mix is an audio input, such as a mixer.
• Local network. If macOS asks to let the app find devices on your local network, click Allow. That's how it reaches the other DJ directly.

A session

• Linking up: one of you makes a code (or invites the other from Find a partner). You both enter it in the app and press Connect.
• What it costs: your account shows your sessions. A session is counted only once you're linked, so a link-up that never works costs nothing.
• How long: 60 minutes, minimum 30 minutes. You're told the length when the code is made, before you start, and it's kept.
• Who's live: one of you at a time. The other hears the live DJ through their own gear and mixes in. While you're live, your partner is silent at your end.
• Handover → Accept: the live DJ presses Handover, and the other DJ presses Accept when they're ready. The new live DJ hears "You have control", and the other hears "Handover complete".
• The delay: in our tests the other DJ's mix reaches you about 0.1 seconds after they play it (about 100 ms through Cloudflare's relay on one connection in England); between two homes it depends on the distance and your internet.
• The jump: when you take over, your partner hears the music jump by about 0.2 seconds; you hear little or none. The voice line covers it.
• Direct or through a relay: the app connects you directly when your two networks allow it. When they don't (some home and mobile networks), it uses Cloudflare's relay. Once you're linked, The link in the app shows the path and the round trip.

No link-up?

• Check that you're both logged in, have entered the same code, and have both pressed Connect.
• Give it 30 seconds. If the app says it couldn't connect, even through the relay, try again with one of you on a different network, and with any VPN off.
• A link-up that doesn't work costs no session.
• Your partner can't hear you: is your DJ software playing, and picked in Your mix? If the app says there's no sound from it, check System Audio Recording (above).
• You can't hear your partner: are you live? Your partner is silent while you are. If not, check Your partner and Partner level.
• Still stuck? Send us the app's log. In Finder choose Go → Go to Folder, enter ~/Library/Application Support/djb2b-app/ and send us djb2b.jsonl.

Honest limits

• You hear your partner about 0.1 seconds late (more between distant homes), so mix in by ear.
• Your connection matters. Losing about 1 packet in 100 is heard as small gaps. From about 3 in 100, mixing in doesn't work.
• Mac only. Sending your DJ software's sound needs macOS 14.2 or later.