The Charlotte Observer is reporting that up to 60,000 people may have found it too difficult to choose just one Carolina to vote in, so they picked both. While the South Carolina race isn't sewn up, the North Carolina race is considered much closer. Depending on how much time people have during the day, this might lead to voters casting ballots in both states, or one of their choice. (Alarmingly, North Carolina doesn't want to discourage voters by asking them to show ID at the polls, while South Carolina does ask for ID.)
I understand that campaigns need to strategically and tactically allocate resources. I just hadn't realized that voters were one of those resources.