Forum Discussion
Multi Forest in trust - New Hybrid Exchange Setup - Only one Exchange Server
Two seperate AD domains (contoso.local, fabrikam.com) which are in a forest trust on the same LAN contoso.local AD domain has an exchange server setup with contoso.com to receive emails - the pla...
Azure AD Connect can certainly handle a multi-forest sync scenario: https://docs.microsoft.com/en-us/azure/active-directory/connect/active-directory-aadconnect-topologies.
Does Fabrikam have an Exchange hybrid in place already? If so, you'll need to be careful about setting up a second hybrid (for Contoso) to the same Office 365 tenant. Certainly possible though (https://technet.microsoft.com/en-us/library/jj873754%28v=exchg.150%29.aspx).
Lastly, if your goal is to remove the Contoso AD entirely, you'll need to either (A) migrate the Contoso AD users into the Fabrikam AD forest and then perform a "hard match" with Azure AD Connect, or (B) convert the Fabrikam users to cloud only in Office 365.
To be honest, this is not going to be simple. I would highly recommend reaching out to Microsoft, a partner, or a migration vendor for assistance or guidance. Best of luck!