Warnings, disclaimers, so forth.
It had all started as a game, as far as they could recall. Alone in their playpen or their toddler bed, when Mummy had been too busy with Percy or the babies, they would babble in unison, mirroring each other's clumsy movements and giggling in stereo. Often, they would trade clothes, driving the whole family mad trying to figure out if the little redheaded preschooler with the F on his shirt was really Fred, or the G was George. Not that it had ever really mattered - they knew who was who, and they were almost never apart as it was.
In later years, their game had grown in complexity, as they'd discovered the concept of counterpoint (not that they'd known the word). They'd learned to move in different, complimentary ways, the yin-and-yang of looping about each other in a mad dance of flying, or the silent stealth of pulling off a trick.
By the time they'd begun Hogwarts, they'd almost gotten the concept of speaking in single, full sentences... and if they'd slipped up and spoken in unison, or lobbed a single sentence back and forth between them every so often, their classmates grinned and laughed and passed it off as being twins.
As the years passed, they'd been in identical classes, played the same position on the Quidditch team, been forced to play as a single unit, and they'd slowly started to slip more often with their speech. No one had noticed.
At the end of fifth year, they'd gotten the same OWLs - not nearly enough by their Mum's standards - but had dropped different classes. But even the single class they didn't share (Potions for George, Arithmancy for Fred) wasn't enough to stop them. They slipped up more and more often, moving in unison, speaking in unison. No one noticed. No one cared.
Then Genkai had come, with her strange students and her talk of core magic, and she'd discovered their intertwined luck-magic. And the whole process had accelerated.
By midwinter, they'd taken to sleeping in the same bed. An elf named Dobby had been most accomodating about replacing the two student twin beds with a single, full-sized staff bed. Their roommates didn't mind. They were twins.
Late one night, scarce weeks before they graduated, they lay awake in the dark, clutching each other with the unthinking desperation of small children.
"A little. But-"
Neither pretended not to know what the other was talking about.
"Do you think we'll-"
"-integrate completely? No. Do you-"
But that wasn't the question on their minds, and both knew it.
"Do you think we can-"
"-survive apart if-"
Alone in their bed, the twins mirrored each other's movements, pulling up the duvet and slowly drifting to sleep in unison.
It had all started as a game.