The two hour timer only increments while you are logged in, you can have double exp after the event is over. Even weeks later if your alts are parked and not played. All you have to do is make sure you apply the buff while the event is still active. Then you can park them and have the Double Exp when you next log in.
Also, if you have an alt with a partial boon that applies the double exp, and you want to park that alt with a full two hours, then: Remove that first in the Character Sheet (right click on the effect if your on PC, not sure on console) then drink the Witch Mothers potion again. This will give you the full 2 hours.
This way, when you log to an alt in the future, you will have a couple of hours of double experience bonus.
EDIT Clarified the Text
Edited by Nestor on October 31, 2017 3:44PM Enjoy the game, life is what you really want to be worried about.
PakKat "Everything was going well, until I died"
Gary Gravestink "I am glad you died, I needed the help"