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

to sugimoto said:

The first question is, can I share between the two Tenant the same e-mail domain during the coexistence phase?

Quest On Demand Migration (ODM)has an optional licensed service, Domain Coexistence, that can handle Address Rewriting between tenants. This will allow either a target user sending as the source email address or a source user sending as the target email address during a phased migration. This meets your technical requirement. 

Quest on Demand also supports the migration of additional workloads other than mail. It can handle the Azure AD accounts and groups, OneDrive for Business, Public Folders, SharePoint Online, and Teams.

Quest On Demand for Email (ODME) only handles email related content. (This is the ODME forum)

Microsoft does not allow the same domain to be registered in two tenants at the same time. So to directly answer your question, no you can not "share" the domain at the between two tenants directly . You have to use an Address Rewrite system to address this short coming. 

to sugimoto said:

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.
(Example.)
At initial migration: aaa@maindomain.com  to  aaa@target.onmicrosoft.com
For differential migration: aaa@source.onmicrosoft.com   to  aaa@maindomain.com

When using ODM and it's Domain Coexistence function. It is recommended not to change the tenant membership for the domain until After the users has switched using the Desktop Update Agent (DUA). If tenant the domain is registered to prior to the switch, the DUA will not work. DUA does not support domain more prior to switch. If domain move prior to switch is required, the Client Update Agent supports this function and can be coupled with DUA to handle the ODB and Teams switches as well. There are other considerations required and steps that need to be executed to support this. 

to sugimoto said:

Are ODM licenses recognized and consumed by UPN?

ODM consumes licenses when you match the source and target user objects. ODM matches on the ObjectID. So you would only require/consume 300 licenses. 

to sugimoto said:

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

Yes, 300 is licenses enough for this migration. 

Dear Jeff,

just for my understanding. 

So I can make

1.) a pre-stage migration in ODM for Emails (f.ex. everything older than 1 month),

2.) switch the domains (manually) between the tenants (like sugimoto has written),

3.) making a refresh/resync of source and target tenant in Quest tenant configuration

3.) making a final migration in ODM for the rest of the emails (f.ex. everything newer than 1 month)

Because ODM is based on UPN's there shouldn't be a break between source and target accounts after domain switch, correct?

Also you have written that "Client Update Agent supports this function and can be coupled with DUA". Is there any KB with details how to do it.

Regards

DN

Dear Jeff, now that is a first.

DUA 2.0 supports domain move prior to switch and the current version downloadable from the project is DUA 2.1.  I will need to check on what the process they followed while testing/QA DUA support for this. Can I get post your question to On Demand forum and I will post the process there. ?  www.quest.com/.../

Dear Jeff, now that is a first.

DUA 2.0 supports domain move prior to switch and the current version downloadable from the project is DUA 2.1.  I will need to check on what the process they followed while testing/QA DUA support for this. Can I get post your question to On Demand forum and I will post the process there. ?  www.quest.com/.../

The process to use DUA to to support a domain move prior to switch is as follows (This assume you have a working migration project)

1. Deploy DUA 2.0 or higher
2. Migrate mail 90 days or old ( or whatever you like)
3. Move the Domain from T2T
4. Run a new discovery task
5. Map all in scope objects, Personally I would use an CSV file with the UPNs. 
6. Switch all mailboxes 
7. Migrate the remaining 90 or newer mail. 

Other workloads are not impacted.