I like both legendaries. Both are a breathe of fresh air from all the past dragon legendaries.
Yveltal's black lines on his body remind me of Groudon.
Xerneas - Ground (horizontal axis)/Psychic (those horns...)
Yveltal - Flying (vertical axis)/Dark
Those types make sense given the design and backstory for the legendaries, though it will mean that Xerneas would be unable to hit Yveltal with either of its STABs. Fighting/Flying would be an interesting type to see for Yveltal; in addition to the Norse mythology basis of the legendaries, the legendaries can be seen to represent the germ layers - muscles and blood for Yveltal, and neurons for Xerneas (with the Z serpent representing the endoderm it would seem). Thus Fighting/Flying would make sense for Yveltal if it is in fact partially based on the muscular system. By the same merit, Electric or Psychic could both work as one of Xerneas's types if it is based on the neuron. Just some food for thought.
Psychic/Electric would be fantastic for Xerneas
It would be a nice type (and it would be a unique type as well, wouldn't it? I can't think of a Psychic/Electric Pokemon) though then its typing would completely disregard the whole Norse mythology basis of its design. I'm personally hoping - and thinking - that its type as well as Yveltal's type will try and incorporate both of the major bases for their design - the Norse Yggdrasil mythology and the germ layer thing.
Gen. III: Ground<Water. The difference can easily be identified thanks to the typings.
Gen. IV: With Dialga being Steel and Palkia being Water, there's a very subtle reference to land and sea, but is basically pointed out as Time and Space. But Dragon=Dragon, so it's a tie anyhow.
Gen. V: Zekrom and Reshiram are obviously weak towards each other thanks to Dragon, but Electric and Fire are rivalled forces, according to Flint and Volkner. I could be wrong, but Dragon=Dragon again. Though Kyurem (which we could reference to "Z") is weak to Fire (Reshiram).
So we could say that, if we had Z along with Xerneas and Yveltal, Z would have a typing weakness to one of those legendaries.
So given the information we already have, Xerneas is obviously land-based and Yveltal is Sky-based. So I'm guessing: