There are two scenarios explained here. The first is switching from a DJ using Scratch Live to a new DJ using Scratch Live. The second is switching from a DJ playing with normal records/CDs to a DJ wanting to use Scratch Live.

1. Scratch Live to Scratch Live.
There is currently no way to work around switching the USB cable from one computer to another without killing the music. Arrange some music to be played from a CD/iPod/normal record that can fill-in the time it takes for the hardware to be switched and the turntables calibrated.

2. Normal records to Scratch Live.
Either play some music from a mix CD/iPod/vinyl record that can fill-in the time it takes for the hardware to be set up and the turntables calibrated. Or, if the DJ currently playing is OK with you setting up Scratch Live around them, you can follow these instructions:

1. Connect your laptop to power socket and switch on.
2. Connect external power supply to the SL 1 unit.
3. Connect the USB cable from the laptop to the SL 1 unit.
4. Start Scratch Live and wait for all the crates to load.
Assuming the current record playing is on turntable 1.
5. Connect the phono cable from turntable 2 to the SL 1 unit and connect the RCA cable from THRU 2 from the SL unit into the mixer.
6. Connect LINE OUT 2 from the SL 1 unit to the mixer.
Assuming the current record playing is now on turntable 2.
7. Connect the phono cable from turntable 1 to the SL 1 unit and connect the RCA cable from THRU 1 from the SL unit into the mixer.
8. Connect LINE OUT 1 from the SL 1 unit to the mixer.

Now the DJ can continue playing their set. Then just before they finish you can step in, calibrate each turntable thats not being used, and you're all set.