Dragon Plays with Fire - S1-E13
Continuity mistake: Danny grabs the keycard off the guard with his left hand. He still has it in his left hand outside the elevator door and when going inside. But it's in his right hand when he puts his hands on the back elevator wall.
Answer: In the comics Iron Fist is able to use the power on both hands. There is no reason why Danny shouldn't be able to. Presumably he chooses to only use the one fist at a time, focusing all his power into one hand rather than distributing it between 2.