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Well the boost is that it returns your energy to 100%. If you use two at the same time it won't give you 200% energy. You cannot go above 100%. However you only have to wait 12 hours to use each card again.

Thank you for the information 😃

I think, what JUX is saying here is that, when you use one card it'll increase the energy to 100%. It's up to the player in how much time he/she utilize that energy, 1 hour, 2 hours et cetera, and then the second card can be used to increase the energy to 100% once again. There is no advantage in using more than one card at the same time, it'll waste the other cards function after the first one if used immediately.

Once a card is used, it can again be used after 12 hours.

Yes, I figured that out by playing the game. Thank you for the response.