We have a working ArcGIS portal version 10.5..1
1. We follow the standard ESRI instructions for setting up a highly available portal.
2. we have copied the content directory to a separate highly available file share with correct permissions
see the screenshot below try to join the second portal with the Primary site. we've also set up network cluster with NLB configuration, the setting has been updated in the portal admin site.( Home > System > Properties)