API documentation exists for Secondary Domains, but the CLI API does not have support for it. This is true for a current InterWorx server will all updates installed.
There is no mention of a controller named DomainsSlave. Can it be added?
Code:
# rpm -qa | grep interworx-cli
interworx-cli-2.0.0-1.iworx.x86_64
# siteworx -u -n --login_domain example.com --help | grep Domains
--controller|-c <string> Index|Overview|Email|DomainsPointer|DomainsSub|Ftp|FtpSessions|MysqlDb|MysqlUser|MysqlPerms|Cron|Htaccess|Simplescripts|Users|HttpdLogs|EmailRemotesetup|EmailSpf|EmailDomainkeys|Backup|BackupSchedule|Prefs|Notice