Warning: Mildly implied sexual content.
Fëanàro worked tirelessly, days and nights, for an entire year under the Light of the Trees, before all three of his Silmarils were made.
In his joy of having created such three divine treasures, so beautiful and so precious—from the fire his own fëa, of whom his mother had given her own to nurture—his wife had been the very first to whom he had revealed his creations.
When she held the eldest gem, he thought she had never looked more beautiful, her coppery-red hair blazing a fiery scarlet in the same away that their eldest son's would. Freckles shown in their lovely constellations across the bridge of her nose, beneath wide gray-blue eyes that now glittering like iridescent silver in their radiance.
He stared until she looked up, the fire between them reawakening as strongly as ever in their youth when he leaned over to kiss her.
They made love in the forge, with the jewel still trapped tightly between their clasped hands.
My theory is that the Silmarils have something to do with the Elves being able to have twins among them, or Fëanor's fëa being a heightened source within the jewels that would allow others who held a Silmaril to increase the odds of having twins, but not always.
Dior and Elwing are prime examples. Elrond is a possibility, but with his mannish blood, and already passed down from a generation with twins, the genetical odds could start increasing on their own at this point, and likely granting the same to Elladan and Elohir, or even Arwen, by the usual 1 out of 200.
