If I recall, double cousins are the offspring of two siblings who marry two other siblings. Example: Judy and Jean are sisters. Ralph and Robert are brothers. Judy marries Ralph. Jean marries Robert. Judy and Ralph’s kids are double cousins of Jean and Robert’s kids.
I wonder if that North Carolina law has anything to do with Chang and Eng Bunker marrying sisters and having 21 kids? That’s where they settled down.