this is the text in news.
the bold part is the reason... there is no X2 sign under experience bar right now, so it doesn't work...
This Saturday and Sunday all the heroes and mercenaries will receive double experience from slain monsters in dungeons.
Heroes will inform themselves when the double experience is active from the (X2) sign under experience bar. The double experience will be announce in advance with in-game news.
Soon to come: Double Gold Drop, Double Crafting Chance and many others!