I am presently living in Denmark and will soon move to Norway. My Danish is at A2-B1 level, depending on the task and the subject matter. I also remember some remnants of Swedish from my school years.
I expect there will be interference from Danish when learning Norwegian.
This website already contains questions that give general tips for avoiding language interference in this kind of situation: How can I avoid confusion or interference when studying two similar languages? , What are some techniques to better differentiate between the two languages? . The most applicable tips seem to be to study the languages with different tools and in different contexts.
What language-specific techniques there are for learning Danish and Norwegian at the same time, or at least for retaining Danish while learning Norwegian?
An example for writing might be: If I know how to write a verb in one language, are there some patterns or heuristics for writing it in the other, too? Something like this could also be true for order of words in sentences, for pronunciation, etc.