Even though you are logging to multiple FortiAnalyzer units, this is more of a redundancy solution rather than a complete backup solution in this example. The multiple FortiAnalyzer units act similar to a HA cluster, since if one FortiAnalyzer unit fails, the others continue storing the logs they receive. In a backup solution, the logs are backed up to another secure location if something happens to the log device. A good alternate or redundant option is the FortiCloud service, which can provide secure online logging and management for multiple devices.