Doesn't matter. It's easier to hang them horizontally for me. Do the
bottom one, then set the top one on top of it, tack it half way up on
each end and in the middle and then finish up.

I think you should butt up the top one first up against the ceiling to
get a tight joint, then do the bottom one. You can leave space at the
bottom since you will put molding anyway and no one will see the space.