I am interested in a two panel file manager, which is configurable, and easy to use with support for plugins.
Where possible open source and currently in active development and stable versions.
It would also be interesting if you have command line integration with the file manager implemented.
My intention is to use it to access documents in two panels side by side, and thus have an easy comparison of documents from separate directories.
If I want multiple panels, I use SpaceFM. It supports multiple panels, including just two.
It is opensource. It is actively developed. It has branches for development and stable. It's free.
Here is an example with three panels:
It even supports plugins.
If you want to see the big list of available plugins, click here.
Suggesting a file manager is somewhat difficult to do, given the number of applications available.
Taking into account that you have asked about Fluxbox, knowing the characteristics of it, my first suggestion would be that you use the Midnight Commander, which is in the repositories.
But reading what you would like the file manager to have, my suggestion would be to use Sunflower.
To install Sunflower:
Open up your terminal and run the command below in order to download the .deb package :
you may want to check if the newly installed package needs any dependency, run the command below:
If you don't like it, uninstall it with the commands:
I estimate that you will receive more suggestions, due to the number of file managers that meet your expectations.
If you keep Sunflower, to keep you updated from time to time you should visit this site: https://sunflower-fm.org/pub/