I'm planning for a tricky (at least for me) migration whereby I have master postgres 10 db from a cloud provider that will not allow for replication user connection. I do have a base backup and WAL getting shipped to AWS S3. I've easily set up a log shipped replica as a hot standby to promote and officially migrate off initial cloud platform.
However, I'd also like to have another warm stand by ready and available immediately for HA after the initial log shipped replica is promoted in the event of a failure I have something immediately available. There are several TB of data so it takes a few hours to generate a replica representing significant risk after database promotion.