For an on demand synchronization process Enterprise Sync requires the AccuRev as well as the mainframe credentials of the user who promotes files to the gated stream.
These credentials are prompted only once per session in Enterprise Developer. The second promote should neither ask for the AccuRev nor for the mainframe credentials. If the Enterprise Developer user is already connected to the mainframe (green background of the connection icons visible in the Remote Systems view), then the Enterprise Sync authentication dialog will only prompt for once for the AccuRev credentials.