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.