Moving dock between vertically arranged displays in OS X Mavericks and later

I have had success with my vertical arrangement by hovering the mouse in the orange areas to bring the dock to the top screen. On the bottom screen you can hover anywhere on the bottom of the monitor.

In your case, where your monitors are almost the same size, it might be a bit tricky to find a spot on the bottom of the top monitor where the cursor wont slip through to the bottom screen and prevent the trigger.

Best workaround I have found so far:

When the spaces are arranged vertically, you can move the dock by switching which display is the primary display.

In Settings > Displays > Arrangement, drag the thin grey bar (representing the menu bar) from the box representing the current display with the dock to the box to the other display's box.

Bar in bottom display:

Drag it to the top display:

This will move the dock to the other display. The other display will now be the default starting point for the dock. It will also switch the spaces, including desktop folders and wallpaper.

[Note: this is essentially the procedure in "Method 2" of this answer modified for a vertical display arrangement:

https://apple.stackexchange.com/a/112829/87288

That answer has a slightly better screenshot of dragging.]