When park hopping their are different ways to use your Fastpasses.
Option 1- Be at park 1 for rope drop, maybe go on a couple rides,then use 3 early Fastpasses. Once completed hop to park 2. Once at park 2 you can pick 1 Fastpass, once used another and so on. REMEMBER the later you pick Fastpasses the less choices available.
Option 2- Be at park 1 for rope drop, ride your must do rides then hop over to park 2. You will have already picked your Fastpasses for park 2 in advance (30-60 days in advance). Once those 3 are used you can go back for another.
Option 3- You think you are spending all day in one park but change your mind. You would have already used your Fastpasses for park 1 so now you are on the option 1 choice. It is certainly possible to do this.