I dunno... looking at the wings in-game and comparing it to the textures, it seems like it just uses the entire texture on both sides.
Hej ~!
Although that does seem like a pretty neat idea, so far, I haven't seen a separate texture for both top and bottom of the wings. It does uses the whole wing texture. The only way to have separate textures ( not for top / bottom ) is to use 2 different textures. Again, it won't be top / bottom but as a whole.. Like half and half persay if that makes any sense. Unless someone came up with such a solution, it's currently not possible.