No they don't stack up you can only use one at the time and to do that you need to activate them first to do that you need to go in game as they can only be activated in game go to the Exit menu in game and in the bottom of the menu you will see a XP Boot icon press it and then choose your XP boost note they only count in game and even when you quite the time only counts in game and when you go to play again the clock will start you can only have one at the time if you choose to activate another before the time of the first has passed you will overwrite it and cancel the first one and lose it every XP boost only lasts 1 hour of game play and the points from them only appear at the end of the match that is all
if this helps pleas give XP to my account thanks
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