Anyways, can we stop this tangent?
Back to the actual topic. I looked back at the cover art again. My concept is simple. The actual environmental background is one type of the legendary while the contrasting shade of the environment is the other type. Xerneas has a forest background, hence its possible grass typing. The shading outside of the X-shape is light thus representing its fairy or light type. Xerneas in would be Grass/Fairy or Grass/Light (I am still mentioning the light type just because I want people to realize that the fairy type is not based on the typical American archetype of fairies- that being tiny girls with wings a la Tinkerbell. Light type covers a larger range and deals with characteristics other than appearance; Xerneas doesn't look like the average "fairy").
Yveltal has a sky as a background, hence its possible flying typing. The shading outside of the Y-shape is darker, which should represent its possible dark typing. Therefore Yveltal could be Dark/Flying, or Flying/Dark.
These two sets of types, Grass/Fairy and Dark/Flying are suitable for both of the legendary Pokémon. Xerneas is similar in appearance to an elk. Sawsbuck is also pretty similar and is grass type. The light or fairy type could be contributed to Xerneas' light crystals, if you will, which are embedded in its antlers. On the other hand, Yveltal is a clearly a Flying type-esque Pokémon, it has wings and resembles a bird or a dragon (I would say more bird than dragon because of the almost down-material coat it has on its neck which is similar to other flying type/bird Pokémon such as Articuno and Honchkrow ). Overall, Yveltal resembles a bird of prey to me because of its overall softer texture appearance that most dragons do NOT have, example being Altaria, but Altaria is also Flying type. Also in regards to appearance, Yveltal's tail and wings are more relatable to flying type Pokémon because its wings do not appear to be webbed like most dragons, and its tail does not come to a point like most lizard like dragons. Yveltal's coloring is, overall, a very dark palette which can represent its dark type.
When it comes to concept stories of legendary Pokémon, many people have mentioned two main theories. One is the Yggdrasil theory, which comes out of Norse mythology. If you read up a little on this theory it is well represented with these two legendary Pokémon, among other potential creatures. The other theory that has been discussed is the creation versus destruction theory. Now, I can't explain everything on here, because I feel that it would be too much to read but I will point out some key supportive ideas that I see.
1. The typings I mentioned at the beginning represent the cycle of creation and destruction. As neither legendary has an overall advantage against the other, it seems to me that they are another pair of opposites. Now although people may argue that this concept of opposites didn't always exist in Pokémon, I can assure you that even Lugia and Ho-oh and Groudon and Kyogre were considered to some degree opposites, just not in typing. Eventually though, Gamefreak made the typing of the legendary duos of later generations to have no significant advantages over the other. This pairing of opposites shows that you need destruction for creation and vice-versa.
2. Xerneas is a good representation for creation and life because of its atmosphere of the forest. Forests, especially rain forests, are known for being extremely fertile and have a high variety of species; in fact, the discovery of new species occurs quite often in those environments. The light crystals on its antlers almost make a rainbow. I think rainbows are also representative of creation because of biblical and literary allusions of them. Also rainbows convey hope.
3. Yveltal's appearance represents destruction with red and black, which are colors that one typically see in a disaster zone (fire and smoke). Also something I thought about mentioning is the red and black colors of the French Revolution (Les Mis). Since this game is taking France as its inspiration, maybe this is an allusion to that? Also red and black are colors of anarchy that were represented in many revolutions around the world.
4. Speaking of revolutions, we need to mention how pertinent they are. A revolution is typically a large movement towards new ideals. Many revolutions, but not all, are violent. This simple fact ties in with the destruction of something for the creation of something else. Sometimes, total destruction is the most effective way of bringing a new concept or creation to life.
Well, I hope I didn't go on for too long, but this is just a summary of what I believe so far.