The F8 hotkey only toggles the main tree on or off. If you want it to toggle both, edit it and change the command from Set TREE=Toggle to Set TREE=Toggle,Dual.
Note that if Preferences / Folder Tree / Options / Open second Folder Tree in dual display mode is turned on then it'll do that automatically and you don't need to do anything else.
You shouldn't edit those files by hand, or even need to look at them.
If you want to update a layout or style, load it, change things in the lister, then save over it. (Or edit it via Preferences / Layouts and Styles, if you want to change one of the settings shown in the UI there, of course.)
In this case I must edit the file "Dual Horizontal.osy" by hand, otherwise I'd have no chance to change the width of the folder tree. In Preferences\Layout and Styles there is no possibility to change it (or am I wrong):
So I added to the file only: "tree_size="270,0"" and, Attaboy I now have the width of the folder tree I wish to have. But thank you, because you gave me the idea to find out how to do it.