Depending on your tattoo placement, you could even do it slightly side by side — much like the Taoist Yin Yang symbol. Which basically means duality, passive/aggressive, etc. There isn't an "accurate" or "inaccurate" way to position them because it largely depends on the scene you are creating with combinations of elements, their facial expressions, and body language. Here's another perspective: your evil side may not be entirely negative, because, without it, there would be no Zenko, no growth. So it doesn't necessarily have to be positioned top/bottom only. Explore that!