If the human body was always 100% efficient, walking up stairs two steps at a time should take the same amount as walking up stairs one at a time.
To go two steps at a time, your quads and glutes need to exert much more energy, wasting a decent amount in the process. Thus the energy per minute usage of two steps is decently higher than one step at a time.
Two steps does however decrease the total amount of time (walking up a set of stairs) enough to the point that walking one step at a time would use more calories.