I going through a lot of confusion lately with the articles in the German language.
Is it grammatically correct to say “In den nächsten zwei Wochen” instead of “In nächsten zwei Wochen”?
What are the other cases where one might omit the article?
It is grammatically correct to say in den nächsten zwei Wochen; it is not correct to omit the article.
In German, the idea is that you are speaking about two definite following weeks (the two weeks following) similarly to English. Other languages may think differently and consider next two weeks definite enough.