Thanks for the answers guys, but I managed to get what I wanted doing something completely the opposite.
I basically needed my boss's phone to have her regular extension / VM, and a 2nd private line going to her Extension's VM.
I managed to do this by setting up the Trunk line on her extension # in the "Non Dial-In Trunks" Form. Seems to do exactly what I want somehow, although I don't completely understand it. I also setup a Multicall line for her extension and that does give her the 2 lines that she needs (in or out) / at the same time. Main thing was that private line that comes in and nobody else can use that.
Her setup does the same thing as before the meltdown we had, but it's not setup at all the same way, but anwyays it works for now.