Hey Rob!
Your idea of a natural theme sounds okay to me (not having read the text I can only assume you are on the right track)! You do need a way to make it relevant for Discovery though. Your over arching theme (Thesis) needs to directly reference the significance of Discovery - Just the fact that lots of the texts use natural motifs doesn't justify it being a main over-arching idea. Perhaps the role of nature in Discoveries, but I'm not sure where you could go with that
Basically, you have two choices for structure. An
integrated approach has one concept per paragraph, with both texts in every paragraph - This is what you suggested. While it does have the potential to be confusing, I personally find that they are more effective
if you pick your paragraph concepts well. If they aren't really different from each other and overlap then it can definitely be counterproductive.
The alternative is four paragraphs, two on each text, with two concepts. It's a little easier to keep this organised but it can result in a more text focused response (which is not a good thing in a conceptual AoS essay).
My advice would be to have a crack at an integrated response, they can work really well when done right
Feel free to run some of your paragraph/Thesis ideas past us if you'd like