Many goalies have practice gloves that have a lot of extra padding in them that would make them difficult to use in a game. Just think about how many pucks a goalie would see in a practice compared to a game. With that in mind, goalies usually have two game gloves that they either swap out every game, every period, or whenever they feel like (just using the second glove as a backup). It's possible Halak uses one every period, or one of them is his practice glove. All different scenarios why a goalie would have two (or three) gloves and it just depends on the goalie.