The Mario Kart Live: Home Circuit software will display a QR code on the screen. When you read the QR code using the camera on the kart, the Nintendo Switch and the kart become connected (paired).
If you own multiple systems, the kart can be connected to a different Nintendo Switch.
Note: Each Nintendo Switch can only control one kart at a time. Players control the kart on the Nintendo Switch system while watching the live video from the kart’s camera on the Nintendo Switch game screen or TV (if playing in TV mode while docked).