I suppose that this is not your case. I you where able to (for examlple) to load balance logs between to FAZ then you would come to a point that some of logs would be in 1st FAZ and the rest in 2nd FAZ where from forensics point of view is not good idea. a.
If you have 2 Fortianalyzer Devices, you can configure Fortigate to push on both devices.
except from these b.
Fortianalyzer has a function of the first Fortianalyzer to be in Analyzer Mode and another on Collector mode . That means that Analyzer pushes to Collector. c.
Another option is that if your FAZ is a VM machine you can have a second instance in suspend mode with the same IP in DR site (via Layer 2 or NAT communication).
Follows FAZ modes comparison/capabilities https://www.fortinetguru.com/2016/03/feature-comparison-between-analyzer-and-collector-mode-fortianalyzer-5-2/