Well remember that Plan B used to be a 2 step formulation before the 1 step came out.
I think it's all a matter of patient preference. If the patient thinks that they may by some small chance forget to take the 2nd pill 12 hrs after the first, then they should definitely go with the 1 step. Also the 2 step should theoretically have fewer side effects, since you're getting less drug at a time.