I have just spent 1 hr going through settings and cannot find where I set the folder tree to split to two trees to match the split horizontal listers. I had it set that way and it changed back to one folder tree for a horizontal split list. Where is the choice for a split folder tree?
Is maybe set TREE=Toggle what you need? It toggles on both trees in a dual lister, normal or vertical view.
I think you want Preferences -> Folder Tree -> Options -> Open second Folder Tree in dual display mode ?
Don't forget to use the Filter in Preferences to help find stuff. I just searched for "tree" 
Thank you .... that fixed it, but for it to work the second view must be closed and re opened. DO is a powerful program, but sometimes frustrating when trying to do a simple thing as this. I know I tried this, but I didn't close the second view and re open, that was the ticket.