FortiManager can manage multiple FortiAnalyzer units, but each FortiAnalyzer must be in its own ADOM. You cannot add a second FortiAnalyzer unit to an ADOM.
For example, FortiManager can contain the following ADOMs: adom-1 and adom-2, and adom-1 manages FAZ-1:
The other ADOM, adom-2, manages FAZ-2:
Following is another view of the ADOMs with FortiAnalyzer units: