HelenaGwyn in that case I think you should be able to use any sync tool (rclone, rsync, whatever) to sync the Calibre library directories on each end.
sean Is it ok or allowed to use rclone mount to have the external library connected to CalibreWeb. The documentation says it needs rclone mount --vfs-cache-mode=full --file-perms=0777 cloud:Calibre_folder local:mountpoint to be able to work.
rclone mount
rclone mount --vfs-cache-mode=full --file-perms=0777 cloud:Calibre_folder local:mountpoint
HelenaGwyn yes, you should be able to use rclone mount.