For my newly created fantasy world (for a self-written RPG) I want to design some not-as-usual animals to give my players some feeling of otherness. One of my ideas is that of a bird-like creature but with four wings instead of the usual two ones we know from our earth. As I know from research (mainly here on stack exchange, especially this thread ), there is enough arguable evolutional potential for such a hexapedal species to develop, as I want to keep the wing-independent feet (unlike for example microraptors ). But now I’m stuck with one problem: I would like to have the wingpairs atop of each other (like in this image) but every scientific approach I can find (here or anywhere else) leads to wingpairs behind each other (like depicted on this page ). My understanding of wing-anatomy and aerodynamics is not good enough to work out if in my case the upper wings and the lower wings would hinder each other while flying and if the needed musculature could grow in the birds body anyway.
So here is my question: Is a birdlike creature with two wingpairs atop of each other scientifically plausible?