Microsoft has announced the availability of the Azure Active Directory Synchronization tool – do not mix with Office 365 Directory Synchronization which supports ONLY one AD Forest .
This synchronization tool helps to synchronize your On Premises AD with Azure AD, covering multi AD forest sync, attributes selection…
This is the first release of this tool and more feature will come.
To download this tool go to http://go.microsoft.com/fwlink/?LinkId=511690
You can also take a look on my previous based on a public beta build http://blog.hametbenoit.info/2014/05/07/office-365-multi-ad-forest-synchronization-with-office-365-installation-and-configuration/