If both URL's lead to the same board and content (so everyone will be able to see both English and Dutch posts and it will look like 1 board) then you can default a domain to a subforum with only 1 license.
As soon as it appears as 2 seperate boards, then it will need seperate licenses.
There are a few threads on vb.com that show examples of both situations.
I want one domain to lead to one subforum, and another to my main board.
But in the navbar there will be a link to both the english and the dutch forum. Does this count as looking like one board?