There was a vote in 1860-1865 that settled most of these issues.
That was to prevent a state from LEAVING “The Union”
Not anything about a state splitting into 2 states.
States Rights were promised when they joined “The Union” and then were denied. That was the real issue- Slavery was just a case-in-point.
The question of secession has not been raised. How does the Civil War have anything to do with this?