The only way I know to do this would be to issue a "NET STOP AvCsMgr" to take the primary Unity server off line which would then force the secondary to start. There is a failover wizard, of course, but it uses COM interfaces to do it's thing and has no command line interface you could leverage - I've pinged the core team to see if I'm missing something but as a command line mechanism I know of nothing more than forcing services off line.
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.