pumpkinfish,
I'm not sure how many days you plan to vacation but for this example, I'm going to assume it is a 7-night/8day vacation.
You would break your ressie into two with 4 "nights" of dining options followed by 3 "nights" without. On arrival day, you will be given a total of 8 sitdown, 8 counterservice and 8 snacks to be consumed over a 5-day period. That last day would also be the "first day" of your 3-night ressie.
Day #1Unless you're arriving at the crack of dawn, you do not need breakfast on that first day. Have lunch at a counterservice, splitting it between the two of you. Enjoy one snack each. You can use two of your sitdown options for a nice meal to start your trip. You're now down to 6 sitdowns, 7 counterservice and 6 snacks with 4 days to go.
Day #2Use your counterservice to split a breakfast between the two of you. It might be a little light but you're going to schedule an early lunch PS at a sitdown restaurant. Use one more sitdown option and order another appetizer & beverage to round out the meal, paying for it out of your pocket. Dinner will be counterservice, splitting another meal between the two of you. You're now down to 5 sitdowns, 5 counterservice and 6 snacks with 3 days to go.
Day #3Make a PS for a character breakfast and use two sitdown options for it. Lunch can be another counterservice split between the two of you. Add a beverage and side and pay for it out of pocket if you need more food. Splurge on 2 snacks later in the day. Dinner is another counterservice meal, using one sitdown option and ordering another appetizer, entrée &/or beverage as is necessary. You're now down to 3 sitdowns, 3 counterservice and 4 snacks with 2 days to go.
Day #4Repeat the options for Day #2, but do a snack as well. You're now down to 2 sitdowns, 1 counterservice and 2 snacks with 1 day to go.
Day #5This is the day you would "check out" of your first ressie and "check in" to your second. It is also the last day that you will be able to use your meal options. Do breakfast on your own. Use your remaining counterservice option for lunch and plan a nice dinner using your final two sitdown meals. Grab two snacks at some point during the day.
This leaves you with your final 3 days without the dining option and you would have to pay out of pocket for those days. Chances are very good that the final day is a traveling day anyway and at best you would only need a breakfast and lunch at Disney.
You can also do this by splitting your stay into 5 nights with the dining and 2 without. The idea is to start your vacation with the dining option and then switch to your final nights without it. It doesn't work well going in the other direction.