When migrating using the same email domain for two Office 365 tenants.


We need to migrate 300 users about, from O365 Tenant (Source) to O365 Tenant (Target).
The first question is, can I share between the two Tenant the same e-mail domain during the coexistence phase?
To explain better, the goal is that during the migration time all users (migrated or not migrated) are able to send messages with the same e-mail domain, the original one.

I am thinking of migrating Source to the current domain and Target to xxx.onmicrosoft.com for the initial migration,
and then doing a differential migration just before the final domain MX switchover.
In that case, the source and target UPNs will be different for the initial and differential migration when using UPNs to Discover.
At initial migration: aaa@maindomain.com ---> aaa@target.onmicrosoft.com
For differential migration: aaa@source.onmicrosoft.com ---> aaa@maindomain.com

Are ODM licenses recognized and consumed by UPN?
If this is the case, do I need double the 600 licenses?

If I Discover&Match with ObjectID, will 300 licenses be enough?

Translated with www.DeepL.com/Translator (free version)

Thanks in advance for your answer.