I assume you're referring to the two options for portrait orientation.
Those come into play when viewing a message, please try, you should see the difference.
The account list / message list remains split in both orientations with both settings, at least for the time being.
BTW, did you know that the exact split point can be adjusted? There is a resize handle next to the "dividing line" between the panels, it flashes into view when opening a message and backing out, then fades out, but remains active even when hidden.