Mystical Mirror of Yin and Yang (Kung Fu Panda: Legends of Awesomeness)
Spatial Scission (Star Trek Voyager)
Coyote Starrk and Lilynette Gingerbuck (Bleach) were once the same being, who split itself into two. Though neither of them can remember which one was the original.
Katen Kyōkotsu (Bleach) is Shunsui Kyōraku's Zanpakutō Spirit, the younger half (Kyōkotsu) being the result of the older half (Katen) having split herself originally in order to hide Nanao Ise's Zanpakutō.
The Seven Evils (Diablo) were born from the heads of Tathamet.
Innocent Buu (Dragon Ball Z) separated himself into Good Buu and Evil Buu.
Mizunes (Soul Eater) from their combined form can divide back into their smaller forms.
Slice 'n Splice (Kirby: Right Back at Ya!) can split anyone or anything into two entities of opposite gender.
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