How can I make sure when I click on the "Dual Display" button that it will open it horizontally and not vertically. I have it set to "Dual Horizontal" in "Lister Configuration" but when I click on "Dual Display" it always opens vertically first and I have to manually change it to horizontal.
The Lister Configuration -> Dual Horizontal is a Style. Styles apply a bunch of saved settings to the current window, but they don't make any permanent changes to your overall configuration and they don't affect what other buttons (etc.) do.
To make the horizontal layout your default, set up a window the way you want it and then use Settings -> Set as Default Lister. The Dual Display button will now open a horizontal display.
Alternatively, edit the Dual Display button and change it from Set DUAL=Toggle to Set DUAL=Toggle,Horiz -- That will make the button use horiz mode explicitly, regardless of the default settings and the window's current state.