Apple - How to get external Apple keyboard to accept modifier key input from Macbook's internal keyboard (and vice-versa)?
KeyRemap4MacBook
I knew I had seen this somewhere…
All you need to do is install KeyRemap4Macbook.
From the "implicit behavior" section:
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.
Yes, it works on other Macs too, not just MacBooks.
I made a utility that does this and works on Sierra.
I hope posting this isn't against the rules, but you can grab it here: https://www.electrollama.net/multimod