It's not a bug, it's a feature.
https://freecommander.com/fchelpxe/en/Viewer.html
Anyway, I agree it's quite strange or even useless to use/have the option "Use the opposite panel" if option "One viewer per panel" is used.
As I see, it's a default option, I think it's not good, it has a sense only for one viewer for both panels, but this way it really looks as a bug.
I think it should be changed, but Marek have to decide.