Thank you very much for your comment and questions. I apologize the delay of my reply.
I think Nitō Ichi Ryū was used by Musashi at the beginning or at the same time as Niten Ichi Ryū. But he chose Niten Ichi Ryū at the end.
Musashi himself said that using two swords is to train both arms. And it is effective going against multiple people as I recall.
In the modern kendo, nitō has become more popular now. Back in old days, some sensei got together and said "we should revive nitō as a part of kendo culture". Then the late Toda sensei, who was famous for his jōdan, stood up (or appointed) to start studying nitō. (Sorry that my memories are pretty bad these days).
Musashi says "If you master thing, you can use the principle for everything." We used to say that we need to learn how to use a sword in chūdan. And then when you can do it well, you can start learning jōdan/nitō. I used to agree with it until I started teaching adults. But that is a different topic here.
I personally don’t really know how to make kendo equipment. All I can do is to show what these craft people do via videos. Hope these vides are helpful to you.
I don’t think I answered your questions but I kind of shared my thoughts, right :D Thanks again for your post!