It's actually more recent. From the mid 1300's the Norwegian and Danish monarchy merged through marriage, and Denmark became by far the dominant party in the union. (Schelswig-Holsten was also for some time part of
The union lasted until 1814, when Denmark (and by extension Norway) had supported Napoleon, and Sweden had joined the other side. Sweden got Norway as a "price". But during that period, a lot of people had started preparing for an attempt at Norwegian independence. And while we entered into a looser union with Sweden, there was a long ongoing debate about creating a new, formalised Norwegian language.
We ended up with two, but one of them - bokmål - was basically Danish + Norwegian pronunciation + various spelling reforms, as this was effectively what was spoken amongst the upper classes in the cities, and this (with further spelling reforms) is the dominant written language today (the other was based on spoken dialects, and is further from Danish but not that much) .
So we've "only" had about 200 years for the languages to diverge again.
The union lasted until 1814, when Denmark (and by extension Norway) had supported Napoleon, and Sweden had joined the other side. Sweden got Norway as a "price". But during that period, a lot of people had started preparing for an attempt at Norwegian independence. And while we entered into a looser union with Sweden, there was a long ongoing debate about creating a new, formalised Norwegian language.
We ended up with two, but one of them - bokmål - was basically Danish + Norwegian pronunciation + various spelling reforms, as this was effectively what was spoken amongst the upper classes in the cities, and this (with further spelling reforms) is the dominant written language today (the other was based on spoken dialects, and is further from Danish but not that much) .
So we've "only" had about 200 years for the languages to diverge again.