Izanagi and Izanami were husband and wife in Shinto religion. Meaning they are a pair, just like Izanagi and Izanami are in the manga. Obito utilized Izanagi against Konan and Itachi stated that Obito (Who he thought was Madara) was his mentor.
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When you actually sit down and think about it, its no surprise that Obito who has Izanagi, would know the pair to it, Izanami, nor is it any surprise Itachi would know Izanami and Izanagi if he were to show it, as Obito was his mentor.
There's also the fact that Madara taught Obito the Uchiha's Kinjutsu, which further shows Izanami is not an asspull.