In fact, the name of the instrument (三味線) literally means three strings.
I thought it would be an alusion that would be clear some time during the movie, and turn out not to have anything to do with the lute and turn out to be a play on words against the instrument name.
But no explaination turned up, that any of us noticed.
Any idea as to the meaning of the title?
On IMDB it shows the names used in all the translations, and most of them don’t keep the “2 strings” name. It’s Kubo and the Magic Sword, or Kubo and the Samari Search. And in Greek it translates back as “two buttons and two strings”.
Besides the fact that most (but not all) languages didn't keep the name, I found it strange that they focused on magic sword. My impression was that the magic was his own (being a demigod) and he used his music to focus and control it. The sword was “indestructible” but not magical in other ways. In fact, it was a point that he decided to stop using the sword to fight with and use his magic strumming instead!
So, I’m suspecting that the 2 strings might be wordplay or alusion that doesn’t translate. Since it is kept in a couple translations that are unrelated languages, I’m leaning toward an alusion or culteral reference.