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:
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.