That's plausible, but there's a gap in the theory. I mean, if both are Indra's reincarnation, then wouldn't Madara's revival have somehow killed Sasuke?
As far as I'm concerned two people can't share the same soul (e.g. in Inuyasha's first episodes, when witch Urasue "steals" Kagome's soul (Kikyou's reincarnation) to revive the later)).
But it's Kishi we are talking about! Kishimoto no Jutsu is a common occurence in the plot 
I don't think it's quite the same, it's like Orichimaru his soul was sealed away by Itachi yet he still left bits of his soul inside all the individuals he took a bite out of, and he seems to be as powerful as ever, so it's possible that the soul of Madara created an identity of it's own even after the soul of Indra moved on, like Madara, I also beleive Hashirama was the previous host of Ashura's soul.
If Hashirama and Madara were not the previous hosts then the question is, who did they decide to be reincarnated as before Naruto and Sasuke? there is a gap of a hundred years and the main two candidates are those two, there is a possability that Yahiko might have been a host after Hashirama but there really is no other, Minato, Negato, Itachi and Jiraiya were all alive when Sasuke and Naruto were born so it can't be them. The Sage clearly said they were constantly being reborn.
One more thing if Madara is the current reincarnation of Indra, then why did Ashura wait a 100 years or so to be reborn (and even then AFTER his older brother died), instead of being reborn a few months after his older brother was reborn. (Sasuke is born in July, Naruto was born three months later.) he'd be leaving his older brother decades to cause havok...It's probably also why the two get compared to the Madara and Hashirama, they were the previous hosts.
Edited by harry4e, 31 March 2014 - 06:36 PM.