There's no automatic failover on a Citadel server, but you can certainly build two of them and simply keep the data in /usr/local/citadel copied from one to the other on a regular basis. You can use rsync for this, or if you have a favorite filesystem snapshot program you can use that too.
You don't have to sync WebCit because it doesn't store any data. This is an excellent method; the backup system can be brought online at a moment's notice, the replication can be performed at any interval you are comfortable with (nightly, hourly, etc.) and there is NO impact on your production system.