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That is a good question, but are you referring to timed boosters or unlimited lives?
I have never bought anything, but generally the response to any question regarding timed anything is that once the timer starts it can't be stopped. This would mean that if you bought six of the twelve hour unlimited lives all of them would run out after twelve hours. Same goes if you bought timed boosters.
Maybe a GM or CM like @Sukanta_Biswas and @Crazy Cat Lad know better.