[Joy]

Anyone who was Half-elven, those with one mortal parent and one immoratal parent, could chose which the became.

Elrond and Elros' parents were Eärendil and Elwing.

Eärendil was half-elven becase his father, Tuor, was a Man (Edain) and his mother, Idril, was the Daughter of Turgon of Gondolin.

Elwing was also Half-Elven. Her father was Dior, the son of Beren and Lùthien. Her mother, Nimloth, was a Elven Maiden of Doriath.

A note on Tuor and Idril Celebrindal - It seems that Tuor - a Man of the House of Hador - was given the choice to sail West with his wife Idril. Tuor was a messenger of Ulmo, so this might have been the reason he could sail over.