First off, let me start by saying I LOVE how you made their hair twist together to make the separation of the yin and yang. It's very creative, and the extra dip in the "dark" Moon's pig tail separates the two as equal visually, even though, from what I've compared, they're probably the same image flipped.

I think you've done an excellent job with the over all drawing skill, although I think in your coloring, you should have done opposite shading on "light" Moon, making it seem like your light source is the Yin and Yang symbol. It just would have centralized the composition better. Other than that, the only thing I see a real issue with is their bangs. They seem abnormally far off their heads but it's more of a nit picky, searching for it thing, than a problem.

Beautiful work. I'm having lots of fun noticing subtle differences in each, such as their eyes, and facial expressions. OOH! And the moons! Nice touch. See? It's exciting.