title_body: How can I explain the origin of the dual number in my Slavic-influenced East Nordic conlang?
upvoted_answer: Both Proto-Germanic and Proto-Slavic had the dual grammatical number. So you could just say that your conlang retained it the whole time. Alternatively you could say that it lost it and then subsequently borrowed it again from one of the Slavic languages which retained it.
  
  If you were after a specific source, Old Church Slavonic could be ideal because it could have a wider influence than other Slavic languages through its religious prestige status. If the religious traditions of the Slavonic Orthodox churches were very highly valued by your conlang's speakers then that would help to explain why the dual number was retained by them even though the other languages of the region had lost it.