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