Elrond knew beyond doubt when it happened.
The day, the hour...the very heartbeat in which one realized it was, in fact, two.
The moment that became an eternity as two frightened grey gazes sought comfort not from him, but from one another, each seeing for the first time not a reflection but a separate being, experiencing the curious sense of lacking that he knew only too well.
Tiny hands reaching for soft blue and grey sacking, tangling in down-soft ebony hair, pulling the other closer, as though in proximity there was balm for their sundered soul.
Elrond knew because he was there.
And their heartbroken cries haunted him still.