How to pair multiple Bluetooth devices?

Switch the keyboard to BT mode. The mode switch is located on the back side of the keyboard.
Select which Bluetooth profile to connect to by pressing the FN key with 1, 2, or 3 key together once to enter the selected Bluetooth profile.

FN key + 1 key > activate BT Profile 01,
FN key + 2 key > activate BT Profile 02,
FN key + 3 key > activate BT Profile 03.

After the BT profile is activated, now you will press the FN key + 1, 2, or 3 key and hold it down for 3~5 seconds. Once the keyboard is in pairing mode, the backlight of the respective key will flash blue slowly.

For example, if you press the FN key + 1 key once to activate the BT profile 01 first, then you will press the FN key + 1 key again, and hold down this time until you see the light flash slowly.
For Mac, navigate to 'System Preferences' and select 'Bluetooth'. find the 'Cascade V1.0' and click on 'Connect'. For Windows 10, go to 'Bluetooth settings' and select 'Add Bluetooth or other devices.' Once the setup wizard finds 'Cascade V1.0', click on 'Pair'. Once pairing is successfully completed, the keyboard backlight will stop flashing. The keyboard is ready for use.