Suppose a native speaker of Swedish and a native speaker of Danish want to communicate. They speak no other languages. (This is rare.) They can read and write in their native languages.
How much could they communicate basic ideas like where their from, where their families live, and what kinds of jobs they do? How much could they communicate complicated ideas like a dispute with a complicated fact pattern?
In this scenario, they have a reasonable amount of time to talk and write it out, but they can't draw from German or English. Neither one lives near the Oresund Bridge where they might get regular exposure to the other language.
I am curious because I speak English, Spanish, and a little Dutch. I understand almost no Swedish or Danish, and the two languages sound similar to me.