What about making her powers of light look identical to his but with light? She can move through light unseen, see in the brightest of light, manipulate light into objects, and make blast of light energy. Anything he does with dark she can do with light. The catch being they have to be within a certain distance from each other to work their powers. And when they work together, their abilities become stronger and may eventually lead them into being able to create a pocket world? 

Their powers could bounce off each other well. He would need light to make shadows and she would need his darkness to define her light. Yin and Yang. And since the symbol itself shows that there is a little of the opposite power in each one, we can even go down the path of him being able to control aspects of light to varying degrees and vice versa.

I'm glad you like the symbol idea! It could be another thing to hide and another reason to come back together. I was thinking it would appear on the inner wrist?