Wonder if this is possible?
We have a client that wants to keep his two domains separate and in different tenants and then sync on prem AD to the two tenants.
The on prem AD would have both domains UPN suffix's added to their accounts in on prem AD as the users in both tenants would be the same.
These topologies of domain to tenant sync are covered well here
You specifically can't sync the same users to multiple tenants.
What are they trying to achieve, there's no reason they can't have multiple SMTP addresses for each user in different domains, what else are they trying to separate ?