Disney doesnt actually recognize "split" stays, but treats them as 2 separate reservations so if you decide to stay at 2 separate resorts (or even room categories within the same resort) any requirements for FD for stay #1 would also be required for stay #2.
FD requires room at rack rate, 2 day tickets for everyone in the room 3 years or older and a minimum stay of 3 nights. Having to purchase the second set of 2 day tickets usually negates the savings of FD.
If you really want to do a split stay with FD for all nights, it is best to get tickets for your entire stay on the first reservation and order just 2 day tickets for the second stay. The 2 day tickets can be saved and used on a future trip - just be careful not to accidently use them at the gate during this trip. A PP said you would have to order 2 sets of tickets equal to the length of the stay (e.g. 2 sets of 4 day tickets), but this is incorrect.
What many people do if they want a split stay is make one res with dining and the other as a room only reservation, hopefully with a room rate discount. If you want FD for the entire trip, then a split stay is probably not cost effective.