So maybe I have to choose, but I am beginning to think that with DOPUS "all things are possible," so I will ask: Is it possible to create a keyboard shortcut to move focus to the folder tree when two listers are in view and Tab has been set to toggle between them? I am not seeing any commands that do that. I know F8 opens the folder tree and Shift F8 opens a second one. Maybe the question should be "do I need the folder tree?", but that is too existential for me right now.
A hotkey bound to Set FOCUS=tree will do that.
There's also a Preferences option to make tab cycle between all elements, not just the left and right file displays, when a lister is in dual-display mode.