Setting up a PBS server on both locations, create a dedicated namespace for each location, or each PVE cluster, (not different datastores so you get a good deduplication ratio) on both PBS servers and then create a sync task on both PBS server so the PBSs could backup each other? So namespace A + namespace B on both PBS and then let PBS A pull the namespace B and PBS B pull the namespace A? That way on both sites the PVE cluster could backup to the local PBS (for fast backups/restores) but you still got a copy of everything on the remote site.
Then you could add both PBS server as a storage to your PVE clusters. And I would set the privileges of the remote PBS so that it is only allowed to restore but not to backup or prune. That way on both sites you could only backup to the local PBS but restore from the local or remote PBS. That way you get some ransomware/user error protection.