The paper face has grain...it is much stronger along the height (or length) of the board verses across the width...I'd run it across the joists to gain the max strength. Run the other way, over time, you may see it droop a little. I'd also probably pay someone to finish what you have rather than trying to attach a new layer on top. That's if a pro would even take that small of a job. The speed and quality they can perform that task is worth it to me.