IANAB, but the way things went down was probably more like this: Two embryos, composed entirely of stem cells, were growing adjacent to each other and bumped in

IANAB, but the way things went down was probably more like this: Two embryos, composed entirely of stem cells, were growing adjacent to each other and bumped into each other, forming a larger embryo composed entirely of stem cells. The stem cells probably aren't really aware that their neighbors have slightly different DNA, so they just cooperate like stem cells do in forming a single human being.