@Bastien-Nollet
The delta backup job saves to srv04-incremental and srv12-incremental.
The incremental mirror job has srv04-incremental as source and srv12-incremental as destination.
The VM that showed no data to copy message was a the "Incremental backup every 4 hours - 8 days retention" backup job.
Originally I only had one remote, srv04. I created srv04-incremental and renamed srv04 to srv04-full and used the mirror backup feature to copy all delta backups to srv04-incremental (as I did not want to attempt to move the data on the NFS backend side). Then, I set up srv12-incremental and srv12-full and created mirror jobs to copy from srv04-full and srv04-incremental. Once the mirror backups were completed I switched the normal backup jobs to store backups on both backup servers.
I can set up a support connection if you want to remotely check this.