I tried this at the command line:

assoc .Folder='C:\Utilities\FileManagers\2xExplorer\2xExplorer.exe'

but it didn't seem to have any effect. Perhaps it would if you followed it with a logoff or reboot. Or... maybe that's not the right association to modify. I dunno. Maybe if you edit

HKEY_CLASSES_ROOT\Folder\shell\open\command

directly instead of using ASSOC? It's set by default to be

%SystemRoot%\Explorer.exe /idlist,%I,%L