Yes, you can have two Xerneas in one battle. You can have a team of six of them if you like, and your opponent could do the same (unless you are playing a competitive battle where species clause would stop you from doing this).
However, unfortunately there is only one Xerneas available in Pokemon X excluding trading as a method of getting it (Xerneas' location may spoil the game so I won't say it). So to get a second Xerneas, you'll have to trade with someone for it, sorry.