On one room reservation, if you wish to purchase a dining plan you must purchase the same dining plan for all members of the party (age 3 or over) for the full length of your stay. On one single reservation, you cannot mix and match dining plans, and you cannot have certain party members have the dining plan for only a portion of the stay - if they're registered on the reservation, they must have the plan for the whole reservation. It's either all members of the party get the dining plan for the whole reservation, or none of them do.
On a split stay with DVC, because there is no ticket purchase requirement, it is possible to book different dining plans without having to spend more on tickets. It's also possible to drop people off the reservation so they won't have a dining plan for part of the stay.
For example, a party staying 4 nights at BLT and 4 nights at AKV can buy the regular dining plan for the 4 nights at BLT and the quick service dining plan for the 4 nights at AKV. The credits they receive on the regular plan will expire at midnight of the day they check out of BLT, so if they have credits left over they'd need to keep their BLT key cards and use those to pay for meals until they've used up all their credits.
The same party can on the same day check into AKV, where they'll receive their key cards with the quick service dining credits on them - these are available for use as soon as they receive the cards and don't expire until midnight of the day they check out of AKV.
So if they have any credits left over from the BLT stay, and they don't use them before midnight after they've checked out, those credits expire and can't be carried further into the AKV reservation. As of the second day of the AKV reservation, they'd have to be sure to use the AKV key cards to pay for meals.