Is multiplayer possible between XCOM Enemy Unknown and XCOM Enemy Within?
Is it possible for a player with XCOM Enemy Unknown to play multiplayer with another player with XCOM Enemy Within?
No, there is no cross-game play between the two. Both players would need to be playing either Enemy Unknown or Enemy Within.
2K community post for reference
Console: Will players of Enemy Unknown Multiplayer be allowed to play together with players from Enemy Within?
No, they are separate Multiplayer experiences.