kirin rei says it was a mistranslation if I remember correctly but only uzumaki + rinnegan can almost equal SO6P NAgato failed because of his illness
Haha, nah. This is the corrected version of it.
The mistranslated version was Tobi calling Nagato the third of the six paths or something like that. <3
@OP He's just talking about those who have the power of the Sage as many others have pointed out.
Regarding those who mentioned that Tobi was the fourth person to do so; you've got to remember that he was in his Madara persona while talking to Konan. That's why he claimed that he was the second person to obtain said power.
Oh, and I'm too lazy to quote, but
@Lamar941; the son of the Sage didn't have the Rinnegan. He had a different, spiraling pattern. It was probably just a watered down Rinnegan, not that doujutsu itself.