You start the upgrade on primary server and at some point you have to start upgrade on secondary server. So at some point during the upgrade, both servers will be out of service. Also when the primary is being upgraded, secondary cannot accept voice messages. Check this link.
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.