I've heard two Tenants and one on-premise domain isn't supported, but when I read this article - it seems to suggest it is ..
We currently have two on-premise domains and two Tenants. We are beginning to use AD for all authentication and having two on-premise domains has been problematic. We will be merging our two domains into one domain. We have no reason to merge the Tenants. We have talked to a contractor and they suggest it would take up to two weeks for users to have their 'old' mail back if we merge Tenants.
Can we not just sync with ADConnect by OU to the appropriate Tenant - each with it's own ADConnect Servers -- we already have the two servers? AdConnect seems to be a verify capable tool. The two domains would have to remain separate - even if we put both in one Tenant -- the emails address are different, and also the Federation methods. Why can't I essentially use the two ADConnect configurations I currently use -- with just minor adjustments for domain name.
People not having their old email and calendar appointments for up to two weeks - is not a good plan. This doesn't necessarily consider what 'might go wrong'.
Multiple Tenants Section
"There's a 1:1 relationship between an Azure AD Connect sync server and an Azure AD tenant. For each Azure AD tenant, you need one Azure AD Connect sync server installation. The Azure AD tenant instances are isolated by design. That is, users in one tenant can't see users in the other tenant. If you want this separation, this is a supported configuration. Otherwise, you should use the single Azure AD tenant model.
In this topology, one Azure AD Connect sync server is connected to each Azure AD tenant. The Azure AD Connect sync servers must be configured for filtering so that each has a mutually exclusive set of objects to operate on. You can, for example, scope each server to a particular domain or organizational unit."