Haruhi wasn't the first one to tell the twins apart.
Naomi Fujimura was a close friend of Hikaru and Koaru.
She could easily tell them apart.
When the twins hit their heads and lost all memories of Naomi, she left to America because her best friends forgot about her.
What will the twins do when their mom lets the little promise Naomi and her made slip?
'Don't tell them about me. I want them to remember by themselves.'
This is going to only be one chapter. One really long chapter. And some other pages with random details of the book like what she was wearing.