One day a toddler named Uchiha Sasuke wakes up with a second person in his head.
This person is himself, and yet not.
Little baby Sasuke doesn't think about it much, he just listens to what the man says and does as he's told.
He trains hard almost every single day after his fourth birthday. Whispers of a coup seem to follow Sasuke everywhere.
Just months before joining the shinobi academy at age six, Sasuke talks to his cousin, Shisui.
More specifically, the voice in his head talks to his cousin Shisui.
(Any anxiety about the man being able to control his body is downsized by Shisui’s use of his mangekyou on a member of the clan.
Sasuke isn't sure which member of the clan, but the older him says Shisui was asked to change the mind of the most influential person in the clan. He isn't sure who either.)
By the time he joins the academy, the Uchiha clan is trying to be more social, to gain the trust of the other clans.
They ally with the Nara first. The clan of lazy geniuses observes the clan of emotional and arrogant social pariahs until they come to the conclusion that they really are just socially awkward.
Then come the Akimichi and the Yamanaka.
After them the Aburame and the Inuzuka, and so on until finally, the Hyuuga become politically neutral toward them instead of extremely hostile.
Shisui pats himself on the back for a job well done.
(Sasuke stays in the shadows where nobody notices him. Sasuke thinks that suits him just fine.)
One day a toddler named Uzumaki Naruto wakes up with a third person in his head.
It's not him, not his angry fox, but a grown man. Himself, he claims, but not.
Himself with nearly thirty more years of life experience.
Himself with a family, with friends, with a home.
Naruto is taught how to conceal his identity in order to earn money, enter stores, and go to the library.
The very last seems to be the most important, to the older Uzumaki.
Naruto is taught to read, to write, to use chakra.
He is taught how to speak to the fox sealed in his stomach, to befriend him.
Then he is told to look for any information on a woman named Uzumaki Kushina, told she might be mentioned in books about the Fourth Hokage.
He finds many mentions of the woman, even finds her name on a memorial stone in a training field.
Naruto is found by an old man. The Sandaime Hokage, specifically.
The man moves him into an apartment at the age of four and begins having weekly meetings with him.
A couple months in, Naruto asks about Kushina. If she is his mother.
His question is answered with a hesitant ‘yes’ and a step back.
Naruto reins his anger in until another few months later when he asks about the fox.
He rages upon learning he wasn't supposed to find out for years.
He is sated by an extra thousand ryo to his bi-monthly allowance.
(With this he can get better food, better training equipment, maybe even an apartment in a better part of town…)
At age five, almost six, Naruto joins the shinobi academy.
He meets a little boy named Sasuke.
Sasuke’s mind yells at him - that's the one, that's him, that's my Naruto, he knows everything - until he finally speaks to the boy in his class named Naruto.
The Uchiha speaks to him.
He's not entirely sure how to respond to the boy’s, “I think I know you.”
Or, he isn't, until the old man in his mind starts screaming for him to tell the boy everything.
The blond boy takes him to the Yondaime’s stone head one day soon after their first meeting and fills his brain with information.
In turn, Sasuke gives Naruto information of his own.
Naruto’s head hurts – there was going to be a coup? Sasuke’s family was going to kill the Hokage? – but he accepts Sasuke’s words with ease.
And he asks Sasuke if he has a voice in his head, too.
Sasuke listens to Naruto’s question, responding with a roll of his eyes and, “obviously”.
Naruto likes talking to Sasuke. His personality is… his personality is like a shock. Like lightning. Like electricity.
Naruto decides he likes Sasuke.
They should be friends.
(The man in his head is grinning. The fox is laughing so loud Naruto almost thinks that if he was a physical being, he'd pass out from a lack of oxygen. The unending amusement of the two confuses him.)
Naruto and Sasuke smile and talk, throwing casual insults at each other every couple of sentences.
(And, despite all the warnings and cautions the time travelers pestered the children with, they were strangely calm as their younger selves spoke.)