Azure AD DS does not support multiple domains its self, however it can sync with an Azure AD that is replicated from an on-prem multi domain setup. AAD DS will just replicate the flat structure of users and groups that has been created in Azure AD.
Azure AD Domain Service Multiple domans

Shola Lawani
521
Reputation points Microsoft Employee
Hello Experts,
I'm evaluating the use of Azure AD Domain Services in Azure.
So the scenario is this...I have an AD Forest with multiple subdomains, these subdomains are currently being synced with Azure AD using AD Connect.
Now I want to spin up a new Azure AD Domain Services instance in Azure that will connect to Azure AD. My question is can an instance of Azure AD Domain Services sync with multiple replicated domains in Azure AD?
Regards,
Shola Lawani