You've got a few options. As another user mentioned, you can use VLT-RCP
to sync content from one instance to another. This, however, comes with
the drawback that it only gets you top-level content, and it will not
synchronize the version history. If you're trying to keep an author
instance in sync, this is not the best option. Similarly, you can set up
an on-modify replication agent on the author to replicate any modified
content to a standby author. This also comes with a similar drawback,
and...