Now I have tested it. Works fine.
So, I have two instances of Google Backup and Sync running. The actual client software installer from Google only needs to run once.
I recall there are some limitations to how many concurrent Google account Google Backup and Sync client accepts. I have tested with two accounts, works fine. Google it :-)
I agree with Chad_Neeper; there’s no way to get offline files, using the Google Drive File Stream client, in the files native format. The files can be cached locally, but in some encrypted rubbish format, not suited for local backup.