Chapter Text
This is how it works: even without that weird ghostly residue of the brothers' chakra, there's a quiet, truer sort of reincarnation working in the background, unbeknownst to just about everyone. Or maybe everyone. Real transmigration of the soul stuff. These souls go from body to body, almost never right after they've died, and often in groups. It's just how it works.
It has been theorised (by some scholars with way too much time on their hands) that this delay in the reincarnation process that has been sometimes observed is perhaps due to the aforementioned chakra echoes manifesting in pairs at various points throughout history. Those chakra signatures only appear after both of their current generation of hosts are dead. That can mean a hefty number of years with only one of the two living. Maybe the afflicted souls get used to that idea.
It's probably just the way that souls like to gather together in familiar patterns.
Here's the thing: give a thousand years, and still statistical probability suggests that it's highly unlikely that the chakra signatures will just happen to follow their previous incarnation to its new vessel. For one thing, souls can reincarnate across family lines, but the brothers' are bound each to his own. For another thing, there's that pesky years-gap dilemma--and Indra always manifests first.
And there's no guarantee that the soul that hosted them previous will be reborn within the proper timeframe. Once both Indra and Ashura's hosts are dead, both manifestations tend to seek out a new host quick.
So yeah, this time's a bit odd. That probably explains why these souls got more than the reverberating subliminal knowledge that was the most that was observed in previously recorded cases of reincarnation. Being able to travel unfamiliar streets without guidance, recognising important figures (to the previous individual) without being introduced, learning techniques unusually quick. (Hey, they had more than a few reasons for being a bit disturbed by Itachi's quiet weariness. An old soul was he, and quick on the uptake.)
Hashirama Senju waited for Madara Uchiha. He must've wanted answers, or just to see whether or not madness fled Madara's soul with Madara's last breath. But he never got his answer 'cause Madara reincarnated basically pronto instead, and, resigned, Hashirama followed suit soon after.
And Ashura and Indra independently chose the same two, because they were the right age or whatever. Who knows how the mind of inhuman entities that technically don't have minds and probably don't qualify as entities work?
That was how it accidentally happened that both the soul and the chakra found themselves reunited in the next host/incarnation. For the first time. And it was in both cases, too! That's bound to screw with the fabric of the universe.
Eh, maybe it'll turn out okay, though?
(Who am I kidding? Madara Uchiha is involved.)