Why do the buttocks have two flaps? 为什么屁股有俩瓣？

Why do the buttocks have two flaps?
by TG

if I were as fatuous as AI
I would fabricate the foot arch as one flat bone
gods did wiser and employed five little bones
cuboid and navicular and three cuneiforms
compact and flexible

if I were as numb as AI
I would fabricate the buttocks as one piece solely
gods did smarter and formatted it into two parts
left and right cheeks and a cleavage between
providing seclusions and letting thighs gap wide