Indra and Ashura are the two sons of Hagoromo (sage of six paths) who wielded the rinnegan. As the two sons of Hagoromo were born they each had a half of his chakra. With one half of the chakra they were very strong but they could not awaken the rinnegan on their own, they needed both halves of the chakra to do so. They could do this by transplanting one another's flesh, as demonstrated by Madara Uchiha (Indra's incarnate), thus combining their chakra to make up Hagoromo's chakra, or they could just receive chakra directly from Hagoromo himself as demonstrated by Sasuke Uchiha (Indra's second incarnate), but it is unknown to why Naruto (Ashura's second incarnate) did not receive the rinnegan, (probably because Sasuke already had it or because he already had his own dojutsu, the sage mode eyes). I mean... If the Uchiha knew it was this easy, they would've raped or even seduced some Senju ladies like Tsuna and had rinnegan wielding babies a long time ago, LOL. Indra and Ashura are the two sons of Hagoromo (sage of six paths) who wielded the rinnegan. As the two sons of Hagoromo were born they each had a half of his chakra. With one half of the chakra they were very strong but they could not awaken the rinnegan on their own, they needed both halves of the chakra to do so. They could do this by transplanting one another's flesh, as demonstrated by Madara Uchiha (Indra's incarnate), thus combining their chakra to make up Hagoromo's chakra, or they could just receive chakra directly from Hagoromo himself as demonstrated by Sasuke Uchiha (Indra's second incarnate), but it is unknown to why Naruto (Ashura's second incarnate) did not receive the rinnegan, (probably because Sasuke already had it or because he already had his own dojutsu, the sage mode eyes).

I mean... If the Uchiha knew it was this easy, they would've seduced some Senju ladies like Tsuna and had rinnegan wielding babies a long time ago, LOL.