There IS precedent for such a split — in Switzerland. Basil city and Basil land split into two cantons circa 1830.
Did you forget about West Virginia? They split from Virginia. The trick is the state legislature has to approve the split and so does the US congress. As the article says it is difficult but not impossible.
That is the reason some CA counties have never split off, to many dems oppose the move. I would love to have a state composed of the Northeastern counties of CA, they are mostly Red. I wouldn't want a straight N/S split because you would have Sacramento and the coastal region included and they are as blue as the sky. Orange county would also be nice to have.