As with the other lengendary Pokémon prior to Pokémon X and Y, both Xerneas and Yveltal has the theme which is the opposite to each other's.
The theme that is represents by both Pokémon is life and death. Xerneas, the living Pokémon, represents life, while Yveltal, the destruction Pokémon, represents death.
There are also different situation when either Pokémon is at the end of life; Xerneas releases all of its life energy into all the living things in around it and transforms into a tree to sleep, while Yveltal steals all of the life energy of every living thing around it before transforming into a cocoon to sleep.
Regardless, both will be in sleep for 1000 years.
The 17th Pokémon movie, Diancie and the Cocoon of Destruction, also features the conflict between the mortality duo.
