Plan to split the checks yourself, because it will be a true exception if you're able to find a server who is willing to split the checks and still give everyone the TIW discount. One thing that works in your favor is that most Disney restaurant checks are printed out with everything organized by seat number. It's just a matter of marking down which seat was which person.
I've found that the easiest way for us to coordinate splitting TIW checks is to charge all meals to the room. (When we're dining at Disney with others, they're usually staying with us.) I have Microsoft Excel on the laptop we leave in our Owner's Locker, and I'll normally just type all the receipts into that at the end of each night while I'm uploading photographs. (I'm a real geek and keep this spreadsheet anyway to keep track of our TIW and annual pass savings each trip. Having the split checks in the Excel spreadsheet is nice because it computes the taxes, extra tip, and TIW discount for each person.) Near the end of the trip, I'll tell everyone what their portion of the room charges came to and they either give me cash, or they can go to the front desk and apply that amount to the room charges using their own credit card, Disney Gift Card, or Disney Visa Rewards Card. So long as you do this settling up before the last night of your trip (or before you reach the approved amount for charging), it allows everyone the flexibility to pay with a card without having to ask the server to charge multiple cards at the restaurant.
If the resort has already processed all the room charges off to the credit card on file, it is possible to get them to reverse this, but it requires the intervention of a manager. It's better to get there before they process the payment in the first place.
