I want to get an Yveltal, but I think someone from Y would like a Xerneas before he gives me an Yveltal.
No. Legendary Pokemon cannot breed since they are in the Undiscovered Egg Group, the only exception being Manaphy which can produce a Phione Egg when paired with Ditto (Manaphy is in the Egg Groups Water 1 and Fairy).
No, Legendary Pokemon are in the Undiscovered group so they cannot be bred. (except Manaphy, buts thats another story)
So, you cannot breed a ditto with Xerneas or a ditto with any legendary(except Manaphy)