Perhaps not such a stunning bilateral one, but two-sex chimerism definitely is: https://en.wikipedia.org/wiki/Human_chimera https://en.wikipedia.org/wiki/Human_

Perhaps not such a stunning bilateral one, but two-sex chimerism definitely is: https://en.wikipedia.org/wiki/Human_chimera https://en.wikipedia.org/wiki/Human_chimera And I can really only say "perhaps". Humans have often hidden or killed their abnormalities for much of history, and birds significantly outnumber us. (edit: there's a semi-recent example of noticable partial bilateral human chimerism actually: https://www.deccanchronicle.com/lifestyle/health-and-wellbeing/060617/woman-has-two-skin-tones-after-fusing-with-unborn-sister-in-womb.html https://www.deccanchronicle.com/lifestyle/health-and-wellbei... Seems like largely a numbers game to me.)