Popping them up at the same time is really good for "pipelining". I know that the year picker is next and I know where it is while I am entering the month. If

Popping them up at the same time is really good for "pipelining". I know that the year picker is next and I know where it is while I am entering the month. If you hide it on a different screen I need to react to it when it appears. Not to mention that you now need a "Next" button and a "Back" button in case I made a mistake (or just want to double-check what I inputted)