Fantasmic! is a 30 min show, no way you can enjoy both shows in one night.
Now some people decide to leave F! early, it's a matter of personal choice but to me, you are paying for the F! package then not enjoying the entire show since you will be on edge ready to leave early, you will inconvenience fellow guests by leaving early (unless you are seated in the very top row on the aisle) then you book it over to center stage. I wouldn't do it. You would need to leave F! by like 9:20pm to be able to make it to center stage in time, also you would need to check into the dessert party at 7pm to get your lanyard so that you have access to the viewing area
They will take people out to the viewing area at 9pm so they can enjoy the new show (9:05pm)