Elros could never tell you how the twin bond works. But he goes to the library as surely as if he knew for certain that Elrond was there. He doesn't, of course. He's not reason to know his brother is here.

But Elrond had always loved libraries.

Perhaps he goes there because it makes him feel closer to his brother.

A shadow, a glimpse, a hissed breath.

And Elros flings himself forwards with a glad cry.

"Elrond!"