You mean the single riders stairs? There wasn't a separate line there and there was only one line (non-express) by the lockers area.
When you make the little zig-zag to enter the castle, if you go straight to enter the queue past the lockers, there's 2 walkway. The one on the left side is chained off - that's Express - and the one on the right is the usual route down the dungeons and outside to the exterior queue. With the addition of Express formally being added to FJ (it used to be on a service recovery, medical need basis only) the Express line has been extended to include the right side walkway down through Dungeons as well, so guests go in, do lockers, go down past the mirror of erised, and then U-turn to enter the Express queue.
It doubles back to the that right-side corner and goes up the stairs to the 2nd version of the Portrait gallery.
Here, Mr. Felton kindly models the 2nd Portrait Gallery in California:
It has that second level landing for starters, but instead of going under all the portaits like in the 1st gallery, you're going up a turning flight of stairs, as seen in this pic of the 2nd gallery in Japan:
Compare it to the good ol' normal 1st gallery:
Edit:
I found a 360 degree video you can scroll around to see the 2nd gallery in action, with people climbing the stairs.