Vertical split panes

My monitor is rotatable and I mostly use it portrait oriented rather than landscape. If I use the split pane function of Thunar (or Nemo) to show two folders side-by-side, they are tall and very thin. It's usable in this format but it would make a lot more sense to have them one above and one below. In this layout, both panes would be almost square.

Given that the shortcut for split pane is F3, perhaps this could be Shift-F3?