Karabiner-Elements modifies following behavior automatically without any configurations:
Share state of modifier keys with all connected keyboards
When you are using multiple keyboards, modifier keys are shared with all keyboards.
For example, pressing “shift key on keyboard1” and “space key on keyboard2” sends shift-space.
Add fn-key combinations
Karabiner adds fn-key combinations:
|fn-up arrow||page up|
|fn-down arrow||page down|