- Connect the first switch to the hub.
- Reset the second switch to the factory settings,
- Hold both switches close together.
- Press and hold both pairing buttons simultaneously – the pairing light will illuminate.
- Press and hold the pairing buttons until the pairing light turns off.
- Enjoy your multi-controller group
This works with up to 3 switches or motion detectors. You can also combine switches with motion detectors.
Source: https://amp.reddit.com/r/tradfri/comments/6x1miq/bind_controller_to_another_controller_to_have/