Maybe Flying/Dark [Y] and Ground/Psychic [X]
Yveltal would then have two immunities to Xerneas. I don't think they would do that.
Maybe Psychic/Steel for Xerneas. At least it would be a Mawile/Sableye thing where one has better resistance with the other having better immunities.
... Just with the weakness to Electric/Rock/Ice... Other having Ground/Fire...
... Just trying to make a point!
