When Arthur is born it's like the whole 'Verse breathes in in anticipation, and everyone in the room can feel the sudden tension. His mother confusedly asks the doctor if there's a second baby. The doctor thinks he should probably be offended by the implication that the best hospital in the Core could miss the fact that she's having twins, but checks anyways. He's just as surprised as she is when the answer is no.
Merlin's a precocious child and his parents are unspeakably proud when he starts talking at nine months, even if they think "find" is a strange first word. His second word is "prince" and that's followed by "magic." They both try to pretend they're not disappointed that it's another month before he says "mama" and "papa."
Arthur's father is working in his home office while Arthur and his friends play "Knights and Dragons" in the next room. He drops everything when he hears screaming and runs to find that Arthur has pushed another boy over and is yelling that he's the prince and he'll suffer a witch to live if he wants. Arthur apologizes sullenly when told to, but refuses to elaborate on why he had gotten so angry in the first place. His father doubts he even knows.
The whole family is sitting down for dinner when Merlin knocks his drink off the table. There's a flash of gold and then the glass is suspended in mid-air. The little town they live in out on the Rim is highly superstitious and they all agree to never talk about the incident again. Merlin promises not to do whatever it is he's done anymore, and it's the first time he can remember lying to his parents.
As soon as Arthur is legally an adult he decides he can't take the itch for adventure anymore. So he takes all the money he's been saving up and buys a used spaceship and hires a skeleton crew. His parents don't want him to go but he begs and argues and declares himself old enough to make his own decisions. Finally, with an admonishment to visit on holidays, they agree and Arthur leaves to find what's out there waiting for him.
Two men from a Parliament sponsored school approach Merlin, offering him a scholarship. They won't say how they found out about his powers but they insist that The Academy is the best place for him to learn control. His parents are thrilled by the offer and encourage him to accept. The next morning all they find is a bed that hasn't been slept in and a note from Merlin saying I love you and Don't try to find me.
Arthur is walking south at the docks in Persephone, looking for a man he was supposed to meet five minutes ago. Merlin is walking north through the Persephone docks, searching for a crew in need of a medic. Then, in a world-shaking event that no one notices, the two bump into each other, sending Merlin sprawling across the ground. Arthur is angry and Merlin is apologetic, but when their eyes meet there's a sudden shift and it's like suddenly everything makes sense.
"Watch where you're going, idiot." Oh, there you are.
"There's no need to be a prat about it." I've been looking for you forever.