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On Linux, I'm using GNOME Commander, and Midnight Commander in the terminal. On KDE, Krusader is the greatest.

For Linux, there’s Midnight Commander.

Gentoo is a great Linux dual-pane file manager, but they should change the name to not conflict with the Linux distro of the same name.

Or Directory Opus. I preferred the version 3 approach with two side by side directories, but the current version is ridiculously configurable.

For macOS, there's Marta, made by someone I know.