If it is anything like how the old KTTK cards worked, you won't be able to use it. I always used to get the extra FP from my wife's card because she is not the thrill ride type. But one day I tried to do 4 parks in one morning thrill ride adventure and ended the sequence at DHS. I rode RnR, and decided to FP ToT since I had to get back and meet my wife for lunch. When I tried using her key for a FP, it said she hadn't entered the park so I couldn't use her card.
Logic would dictate the same scenario would be true with MC. Only difference is that FP+ are made ahead of time. Since the FP+ stored on the MB is similar to the old paper FP, would it matter? Or would the fact that the person the MB is tied to has not entered the park preclude that person's FP+ from being used? I am guessing on the latter. It will be interesting to see what happens.