Look at Kansas City, for instance:
https://www.google.fi/maps/@39.0729204,-94.5932628,12.75z
There border between the states of Kansas and Missouri runs straight across Kansas City. There are two Kansas Cities with separate local governments each governing their own half of the essentially the same city. The population of the city in Missouri was about 460,000 in 2010. The city on the Kansas side of the border has a population of 150,000. The metropolitan area has a population of over two million.
There are many such cities in North America, some existing on both sides of an international border.
Take a look at this list:
https://en.wikipedia.org/wiki/Twin_cities#North_America
There are international borders running across neighborhoods or even individual houses in Europe, too. For example, Belgium and the Netherlands have enclaves inside each other. I've heard that weirdness is the result of land ownership patterns dating back to the Medieval period.
Follow the link, and you'll see a house split by the border between Belgium and the Netherlands.
I've heard that house has two addresses, one Belgian and one Dutch. Tax season must be nightmarish for the people who live in it! LOL
All regional borders in Finland are in the middle of nowhere. The only twin town that has an international border going through it is Tornio/Haaparanta, the latter of which is a town on the Swedish side of the Tornio river on the. The regional borders are often in watershed areas. River valleys and systems of interconnected lakes (unless too large to be one region) typically form a region.