Actually, some consider North Dakota and South Dakota to be "twin states" because they were admitted to the Union on the same date and share a border and a name.
However, the states are also different and have not taken to being called "twin states", so the term is very rarely used.
It is an analogy. North Carolina is to South Carolina as North Dakota is to South Dakota.
North Dakota is north of South Dakota.
The US state of North Dakota is north of South Dakota.
North Dakota is up by the border of Canada, Wyoming is just next to South Dakota.
Saskatchewan and Manitoba, Canada are to the north of North Dakota and North Dakota is to the north of South Dakota.
The US state of South Dakota is south of the US state of North Dakota.
North Dakota South Dakota North Carolina South Carolina
The state of North Dakota is north (or above) the state of South Dakota.
The state of South Dakota is south of North Dakota.
North Dakota is bordered by South Dakota to the south.
South Dakota is further north than North Carolina.
North Dakota is north of South Dakota. If it was above South Dakota, it would be floating in the air.