Thanks, w7lwi.
The first usage (border before) was clear to me, however in the second (border in the end) it was not and I've seen both forms (the ones I wrote) in the Internet, and in fact "Namibia South Africa border" gets more results in Google than "Namibian South African border".
I would say "Namibian - South African border" was correct because "Namibian" and "South African" are adjectives to "border", but maybe I'm using Portuguese logics to explain my reasoning, and in fact in Portuguese, although we would say "Namibian border", we would also say "Namibia - South Africa border" - I just don't know why.
Regards,
Adelaide