"I see the border between the countries."
Translation:Ik zie de grens tussen de landen.
That would suggest something physical in between the countries, e.g. het Kanaal ligt tussen Groot-Brittannië en Frankrijk in = the English Channel is in between Great-Brittain and France. A border is not something physical (a line is not three dimensional).
More in general:
Tussenin (adjective) can only mean something in between objects or in time between things, e.g. de vrouw staat tussen twee meisjes in = the woman is standing in between two girls, ik zit tussen twee banen in = I am in between two jobs = I am unemployed
Tussen (preposition) can roughly have all the meanings tussenin has (but is less defined e.g. de vrouw staat tussen de meisjes = the woman is standing amongst the girls), but also any abstract relationship like tussen hen gaat het goed = their relationship is well/they get along well (lit. between them it goes well), tussen die berg kleren zit vast iets schoons = amongst that pile of clothes there will probably be something clean, de buit werd verdeeld tussen de criminelen = the loot was split amongst the criminals, het verschil tussen geniaal en gek is miniem = the difference between genius and crazy is minimal/tiny.