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 it won't be an exact copy of the primary device either, added to the fact that it's easy for the primary author's replication agents to get backlogged and then your secondary device is out of sync.
Another option is the AEM Cold Standby author product, which has, as its express purpose, keeping up to date an exact copy of the primary device for Authors. It's been known to be brittle in the past, but it does get the job done.
The other thing that many companies will do, for instances that are running in the cloud, is to simply take a snapshot of the primary device and paste it over onto a cold standby device that you could stand up at a moment's notice. You don't get real-time updates with this, HOWEVER that can be a blessing as well. Many of the issues that will take down and author environment (or really create a "mission kill" of an author environment) are unintentional deletions or mass tag corruptions or other such user-initiated items that have nothing whatever to do with a physical outage. Such issues would be replicated automatically if you have an auto-replication setup, but if you're taking snapshots once every [interval] then you have [interval] time to refer to the backup.
More on this subject is here: https://www.opsinventor.com/on-choosing-a-disaster-recovery-strategy-for-adobe-experience-manager/
Hope that helps!
