What you are asking for is tricky, but possible. You can setup two repos, one for each server. On the first server (in datacenter A), you have a "normal" pgbackrest configuration, with an explicit backup-standby=n. pg1-* points to the local server, pg2-* points to the replica. The repo is in datacenter A.Backups run from PG server in datacenter A, to the backrest repo in datacenter A. WAL is archived from A -> A
On datacenter B, you have a separate local repo, a different stanza name, and within that stanza, you have backup-standby=y, pg1 pointing to the local pg, pg2 pointing back to A. In the postgresql.conf, your archive-command pushes to this second repo, and archive_mode is "always". Backups run from the PG server in datacenter A, but 99% of the files are actually copied from the replica in datacenter B, and goes to the backrest server in datacenter B. WAL is archived from B -> B.
That's the basic idea. It falls apart rapidly if this second data center ever fails over.
