Forum Discussion
Azure AD synch and different Azure AD tenants?
The above is reasonable advice, but there is an important correction to be made depending on what it is you're looking to achieve.
Your final question appears to be: "can the two on-premise directories (original. and dev.com) be consolidated into a single tenant ("Azure AD first")?
While the answer remains "yes", the correction to the above advice is that you cannot (it is not supported outside of something called staging mode, which isn't going to help you) run multiple Azure AD Connect instances pointing to the same tenant.
Instead, you will have a single Azure AD Connect installation that points to both original.com and dev.com on the Active Directory side, and "Azure AD first" on the Azure Active Directory side.
As some of the comments above imply, this needs careful planning but all I wanted to address here was the possibly-incorrect assertions from above that you would run AAD Connect installations in each Active Directory forest, as if I've understood the final questions correctly, that is untrue.
What you do want to do:
What you don't want to do (i.e. separate AAD Connect installations per forest to a single tenant):
Cheers,
Lain