If you only pre-book one or two FP (only one can be tier one) you can book another tier one but only on that day and only in a different park. Which isn't that different than pre-booking three FP and using a 4th for a tier one, but it does allow you to book the second tier one earlier in the day if you plan to park hop.
Ex: Book a FP for Test Track at Epcot for Sunday, March 11. On Sunday, March 11, you can scan in at Test Track and immediately look for and book a FP at RnR at DHS, assuming one is available.