After a computer crash I have re-installed Dopus after a few years. When I click on Dual Vertical display I get one folder tree and two windows displaying contents whereas I want two folder trees with each displaying the contents of the chosen folder.
Preferences / Folder Tree / Options / Open second Folder Tree in dual display mode
Thank you so much Leo.
This was also a big help to me as well, its actually the expected behaviour for new folder views for me, so when it only has one folder tree, it becomes a bit confusing. 