AAD Connect can synchronize the same users, groups, and contacts from a single Active Directory to multiple Azure AD tenants. These tenants can be in different Azure environments, such as the Azure China environment or the Azure Government environment, but they could also be in the same Azure environment, such as two tenants that are both in Azure Commercial.
Important points to consider :
The same Source Anchor can be used for a single object in separate tenants (but not for multiple objects in the same tenant)
You will need to deploy an AADConnect server for every Azure AD tenant you want to synchronize to – one AADConnect server cannot synchronize to more than one Azure AD tenant.
It is supported to have different sync scopes and different sync rules for different tenants.
Only one Azure AD tenant sync can be configured to write back to Active Directory for the same object. This includes device and group writeback as well as Hybrid Exchange configurations – these features can only be configured in one tenant. The only exception here is Password Writeback – see below.
It is supported to configure Password Hash Sync from Active Directory to multiple Azure AD tenants for the same user object. If Password Hash Sync is enabled for a tenant, then Password Writeback may be enabled as well, and this can be done on multiple tenants: if the password is changed on one tenant, then password writeback will update it in Active Directory, and Password Hash Sync will update the password in the other tenants.
It is not supported to use the same custom domain name in more than one Azure AD tenant, with one exception: it is supported to use a custom domain name in the Azure Commercial environment and use that same domain name in the Azure GCCH environment. Note that the custom domain name MUST exist in Commercial before it can be verified in the GCCH environment.
It is not supported to configure hybrid experiences such as Seamless SSO and Hybrid Azure AD Join on more than one tenant. Doing so would overwrite the configuration of the other tenant and would make it unusable.
You can synchronize device objects to more than one tenant but only one tenant can be configured to trust a device.
Each Azure AD Connect instance should be running on a domain-joined machine.
Visit https://docs.microsoft.com/en-us/azure/active-directory/hybrid/plan-connect-topologies for more details