"How long have you known each other?"
Translation:Hvor lenge har dere kjent hverandre?
It the assumption here you are speaking to the two people who know each other, and therefore, only "dere" is acceptable? Because if you were talking to one of the two, wouldn't you use "du"?
You wouldn't ask this question to one person. If you were asking one person you would say "Hvor lenge har du kjent X"(How long have you known X).
'each other' also indicates that the 'you' is plural, as you cannot use it with singular pronouns: "I know each other"(wrong) vs "We know each other".
Unless this is something specific to Norwegian, I disagree with the first paragraph. It's perfectly natural to ask a single person how long they and a second person have been acquainted with each other, even if that other person isn't present.
The second paragraph is key; there's just no relation between who you're asking the question to, and the subject of the sentence. This works exactly the same in English; you just can't tell because the pronoun is "you" either way.