If you are asked for a credit card when you make a reservation it will be charged immediately for dinner shows such as Hoop De Doo, CRT, Spirit of Aloha etc. For other restaurants such as Akershus or California Grill, etc it is put on file as a gurantee that if you don't cancel within a specific timeframe and don't show up, then a no-show fee per person will be charged. The reservation page will tell you which of these 2 situations apply when you make the reservation.
Once you get to the restaurant, let the server know you will be using credits from your dining plan and if your credit card was charged, you will be refunded.
You mentioned that you are booking a split stay with the dining plan on the second reservation only. Just so you will be aware - you cannot use any dining credits until after you have checked into the reservation that includes the dining plan.