"How long have you known each other?"
Translation:Hvor lenge har dere kjent hverandre?
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.
I initially agreed with you but on second thoughts I'm not so sure. It's the peculiarity of English having no separate word for 'you' whether it's singular or plural.
If you ask a person how long they have known someone else, using the phrase 'how long have you known each other?', although it sounds like you are addressing a single person what you're actually saying is 'how long have you two (or however many) known each other?' In which case you are addressing more than one person even if only one of those people is actually there.