Currently there is no way to see the contents of a directory and a subdirectory in the same window.

My suggestion is to add, for directory entries, an icon to the left which allows expanding to show the contents of the directory (and similar for subdirectories) in the same window as the parent, at the same time.
This would be a tree view including all files, not just directories.
I would continue to display shortcuts in the left side panel.

This can be very useful, especially for subdirectories with very few elements.
One could expand a number of subdirectories and readily visually compare their contents, without opening an number of different windows.
(This is something that I have done frequently with nautilus, which unfortunately has become unstable, as well as loosing features I use.  Generally I find Thunar is better laid out.)

Since many other users may not be interested in this feature, I can understand it being made an option, which I would always activate.