Hacking two keyboards together requires making compromises between what is available on the controller and which keys are present on the keyboard.

I got somewhat lucky by buying a keyboard with a Windows compatible power key, so I was able to wire that to the power key on the keyboard.

However there were a couple of other compromises that I made to make it all work properly:

Modifier Keys

Apple keyboards have their modifier keys arranged as Ctrl, Option / Alt, Command / Apple and PC keyboards have them arranged as Ctrl, Windows, Alt.

As a PC user, I made the decision when hacking the matrices together to ignore the labels on the keys and wire the modifiers in the PC fashion. Thankfully, this isn't a big deal as QMK lets me map the keys however I like.

Keypad Equals Key

Numpads on Apple Keyboards have a "=" key next to the numlock / clear key and all of the operator keys (+, -, * and /) get moved around to fit it in.

This isn't something that's done on PC keyboards so the controller I used didn't have any sensible place on it's matrix to wire this to.

Consequently I made the decision to make it work with most calculator applications so I wired it in parallel with the numpad enter key.

This is something that I'll have to rectify when re-wiring the matrix.