This has been a long awaited capability.
As you know until then you were not able to synchronize your Active Directory with multiple Azure AD/Office 365 tenant.
Well, this is now possible and supported to do so; meaning you can synchronize your users, groups and contacts from your Active Directory to different tenant (aka duplicating them).
Before you start jumping to implement you need to know the following:
- You need one different instance of Azure AD connect for each tenant you want to synchronize with; Azure AD Connect does not allow (yet?) to synchronize with multiple tenant
- Only one Azure AD tenant sync can be configure for write-back (groups and/or devices) as well as hybrid Exchange. Password writeback can be enabled with the different tenant
- You can not reuse the same custom domain(s) across each different tenant; unless you are using different Azure environments (commercial, government)
- Devices can be synchronized with multiple tenants but only one can be configured to trust the devices
- You can not enable hybrid experience like Seamless SSO and Hybrid AD Join to more than 1 tenant
The complete documentation cab found here Azure AD Connect: Supported topologies | Microsoft Docs