I was annoyed that Thunar still has no setting to set the splitview permanently.
So I searched for a solution and also found which I would not like to withhold you now.
For this we need to create at least 1 entry with xfconf-query. I have found a 2nd setting which I will also would like to explain also immediately.
All settings I describe here for the console input it works but also graphically with the
“Settings editing” in the settings of Xfce.
For this we have to create the entry “/misc-open-new-windows-in-split-view” in the Chanel Thunar with the value type “bool” and the
value “true”.
- create and activate splitview entry.
xfconf-query -c thunar -p /misc-open-new-windows-in-split-view -n -t bool -s true
From now on Thunar should always start in splitview.
- additionally the splitview can be done horizontally.
xfconf-query -c thunar -p /misc-vertical-split-pane -n -t bool -s false
I set the value here to false because I like the vertical split. Otherwise just set it to true.
-
if you don’t need a horizontal scrollbar in the view and you have a mouse with two wheels you can also use the second wheel
to change the folder levels. This setting is limited to Thunar.
xfconf-query -c thunar -p /misc-horizontal-wheel-navigates -n -t bool -s true -
list channels
xfconf-query -l -
list a channel with properties and values. Example:Thunar
xfconf-query -c thunar -lv -
invert / toggle values Example: /misc-vertical-split-pane
xfconf-query -c thunar -p /misc-vertical-split-pane -T
Attention: I found out that the settings in Thunar in the tab “Behavior” under “Tabs and windows” do not support the split-pane.
“Tabs and windows” can influence the splitview. I have only set “Show full directory paths in tab title”.
in the tab title". The setting “Restore tabs on startup” breaks the splitview again.
But I can live with that. 
That was it. I hope I could provide a little interest or clarification.