The OPENINRIGHT argument really means "open in right or bottom". It'll use whichever orientation the window is currently set to.
"Left" and "Right" are the canonical sides which most command arguments and discussions use, but their meanings change to "Top" and "Bottom" when in the other orientation.
You can switch the orientation using the buttons on the default toolbars (next to the "Lister" menu), or by clicking the = or || icon near the top right of either dual file display panel. The window will then use that orientation until something changes it.
Depending on other settings, you may need to use Settings > Set as Default Lister to save the change. If you're using layouts, those will also need changing.
(I don't think any of this changed in recent updates.)