Umm ... what makes you think that Thunar has the option to show dual panes? It's a fairly basic file manager and has fewer features than some others: that's why it is favoured in lightweight Linux distros. If you want dual panes I suggest you install Nautilus.
Thunar doesn't have this functionality, sadly instead they opted for tabs, you can open a new tab with CTRL+T, you can also drag files on tabs.
Umm ... what makes you think that Thunar has the option to show dual panes? It's a fairly basic file manager and has fewer features than some others: that's why it is favoured in lightweight Linux distros. If you want dual panes I suggest you install Nautilus.
Installed nemo
. Problem solved.