Swashbuckling swallows

Gotham, meet Wilde and Coco

Okay, so you have fraternal twins, where two eggs are fertilized by two sperm in the same womb, and identical twins, where one egg is fertilized by one sperm, but then splits into two cell clusters.

But there's a third category, where one egg and one sperm mix, but then divides unevenly. So you have basically identical twins, except for one tiny difference, which sometimes happens to be an entire chromosome.

So let's say you have a cell with too many chromosomes to begin with. For example, those for gender. Rather than being XX or XY, you have an XXY cell. If the cell divides into two, and rather than duplicate the extra chromosomes, each cell grabs one, you can end up with twins that are genetically identical, except one of them is a girl, and the other is a boy. How often does this happen? Only three confirmed cases in humans. Ever.

Thus the inspiration for Wilde and Coco. Near identical twin swallows who take advantage of their androgynous similarity to confundle the world. The only way to tell them apart without getting them to strip (not a challege, really), is to wait until they speak, and tell them apart by their gender specific voice. Fun!

Oh, and they carry twin shortswords, both named Pinfeather.