It's pretty messy and would need to have some constitutional support drawn up. E.g. you probably wouldn't do it by county per se, it might be better to make it work by congressional district.

County borders make much more sense. Congressional district boundaries change much more often. They also mean nothing to people in terms of their geographic identity. No one says, "Oh yeah, I live over in the 13th District."

A properly drawn congressional boundary should have a predictable number of citizens, however. Arbitrarily choosing county boundaries (and why county boundaries instead of some other method?) would probably upset the balance of population and require reallocation of representatives. Or maybe the price of seceding from your state is that you don't get to take the representation with you and therefore must wait until the next census to clear it up.

I mean, stuff like school board and police departments are often organized by county, no?