Forever
It was never a question that they would spend the rest of forever together. Very early in life they'd been told, no-one could tell them apart. Sometime during that childhood they ceased to be two. Merging into one identity, even names didn't matter anymore. No-one else knew, so they forgot as well, co-existing as one.
But then she had come, like a flight of birds and changed all that. She decided who was who. She even gave explanations.
"Hikaru's actions are one level meaner than Kaoru's."
It didn't mean anyone else could tell them apart. But now they could tell each other apart. So forever was just that much brighter.
Lilly: You know that is sooo not my style…
Laurry: What Gushing incest??
Lilly: heh.. =_='