Xander and Xavier.
Xander because it feels as though it's the correct spelling in my opinion and because visually, it's most appealing in my eyes.
Xavier because it's the traditional spelling and it's very rare for me to prefer a different spelling to the original, and also because Zavier looks and sounds like a whole different name to me.
Yep... Both x's.
Another vote for X&X.
Both X's... The letter Z is just not very appealing.