You can create multiple realms, each connected to a separate domain.
Each realm would need to be referenced in a distinct identity rule.
Reference:
http://www.cisco.com/c/en/us/td/docs/security/firepower/621/configuration/guide/fpmc-config-guide-v621/realms_and_identity_policies.html#task_C63B9C2CD6AA42448371578323FB4E70