Reports are ADOM-specific (i.e., can only be run against data in a single ADOM).
However, you could either use Log Fetch to collect all the logs into a single ADOM on another FortiAnalyzer. Or, if add all FGTs into the same ADOM on an upstream Analyzer, you could use log forwarding to that upstream FortiAnalyzer. Both solutions requiring having 2 FortiAnalyzers.
The Fortinet Security Fabric brings together the concepts of convergence and consolidation to provide comprehensive cybersecurity protection for all users, devices, and applications and across all network edges.