.
I haven't been able to get the Function commands right --
I'm actually trying to configure the R-click functionality of a "Three buttons" mode,
but I think that's irrelevant because the command should work in other contexts as well:
I want 'Desktop' to open in a [2nd] dual pane, from a single panel mode.
I experimented with setting Prefs > File Displays > Options >
"Specify initial folder when switching to dual file display" > Folder > desktop --
but I don't want that behaviour ALL the time.
I've tried these --
Go /desktop Dual=on
Go /desktop Dual=toggle
Go /desktop Set DualFileDisplay
The command-lines you were trying were using arguments that are for the Set command which won't work with the Go command. Every command has its own arguments. (Sometimes the same arguments will work with more than one command, but only if the manual says they do.)
This might work better in case the button is pushed when you're already in dual display mode:
Go /desktop OpenInRight
Set FOCUS=Right
There is also Preferences / File Displays / Options / Set new window to source when switching to dual file display which can be turned on if you always want the dual display to become active when it is opened. If that is on then the second line isn't needed at all.