The easiest way to have a standby backup Citadel is to build two of them and simply rsync your data directory to the backup server periodically. If you have an Easy Install system it's /usr/local/citadel ; if you're using the debian packages it's /var/lib/citadel and /etc/citadel. 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.
Another method is to build a high availability Linux cluster and put Citadel on a DRBD partition. Simply fire up Citadel on whichever node is the active one, and all changes will be replicated to the other node in real time.
Supporting either of these is outside of the scope of Citadel support, but we know from user experience that they both work quite well.