Combine two office 365 tenants - need help

Thank you for quick reply, here are answers:

1. I do not want to setup hybrid, I mentioned it because I heard that is the only option for such migration?
2. When Y.com users will be moved to X.com tenant, their email addresses will be changed to X.com as primary but Y.com will be added as alias.
3. I'm really not sure, what are the options here?

This really goes beyond Office365. This involves the merging of the computing resources of 2 companies.

I would assume, that both independent companies would have

1. Separate Authentication Servers (In this case each have their own AD Servers)
2. Separate IT policies and naming conventions
3. Separate Line of Business Apps (Finance, HR, ERP, Project Mgmt..etc)
4. Separate Security and Backup Systems
5. Separate Networks
6. Separate Productivity tools (Emails, Intranets, etc)

I would start by doing a detailed discovery and mapping out the IT systems of both companies and their dependencies.

Then decide how the merged environment would look. 

The discovery might also reveal how touching one system might affect the rest of the systems in the company.

Getting an experienced vendor/consultant to help you with this might be the way to go.

Thank  you Vincent for detail, you are right both are two different companys with their own ADs, domains and applicatons but both have their on-premise ADs synced with Office 365 tenants (their owns) for 365 emails. For now main concern is to move their emails into primary company's tenant and during this migration we do not want to distrub their computer logins, and cannot afford re-configuring outlook profiles for whole company. So what are the suggestions?

Hi MSITGEEK,

If these two tenants didn't set up Exchange hybrid and the mailboxes are pure hold online, you can follow the steps in this following article to migrate mailboxes from one tenant to another. And based on my experience, if you have no intention to set up Exchange hybrid in the future, there is no need to move mailboxes to on-premises Exchange servers. You may just follow the detailed steps in this article to complete the migration. And there are some things you do need pay attention to: 1. Please back up all your data such as use Outlook's export function in case data lost during the process. 2.To decrease the business impact to minimum, perform it during the weekend.

Besides, I'd like to confirm some information to further help you.

1. Would you like to set up Exchange hybrid or you are hybrid scenario now?
2. Would you like to use X.com as your domain or use both X.com and Y.com?
3. Would you like to manage users in  separate ADs or under X.com AD?

Reference: https://support.office.com/en-us/article/how-to-migrate-mailboxes-from-one-office-365-tenant-to-another-65af7d77-3e79-44d4-9173-04fd991358b7

Regards,

Leo

Hi MSITGEEK,

No, hybrid migration is not the only option, the article I provided above doesn't use hybrid migration. To help you better understand the differences between these, I will explain more: For hybrid migration, you can imagine it like using an Exchange on-premises server as a bridge ---- after it is setting up, emails can be routed to on premises servers, which enables you to cut off the online tenant with a low mail flow impact. Then you will gradually migrate your on-premises mailboxes to targeted Office 365 tenant.

The advantage of the whole process is it has a low impact to your mail flow, and on the contrary, as you can see in the article I provided, after changing the MX records of your domain, it may have a down time to restore according to TTL value and DNS transition time.

But with hybrid migration, it may have additional fees if you doesn't have Exchange server before, for example, you will need to buy licenses for using this service. And you can see the advantage of another method now ------ it may have lesser cost than hybrid migration and it is easier to perform, without the steps to move your mailboxes between on-premises and cloud.

So it depends on you which migration method you want to use, if you don't have a high demand for interrupt of mail flow, the method above seems a better option for you, otherwise you can choose hybrid migration.

Regarding the AD management option, what I suggest is using the method in the above article if you are not familiar with separate management, or you can use soft match to separately manage the two ADs.

Besides, please check Vincent's reply about carefully evaluate your services. Hopefully you can get your work done without encountering any issues during the process.

@Vincent, thank you again for helping our customers.

Some references:

1. Exchange licenses: https://products.office.com/en-sg/exchange/microsoft-exchange-server-licensing-licensing-overview
2. UPN soft match: https://docs.microsoft.com/en-us/azure/active-directory/connect/active-directory-aadconnectsyncservice-features
3. Hybrid migration: https://support.office.com/en-us/article/use-minimal-hybrid-to-quickly-migrate-exchange-mailboxes-to-office-365-fdecceed-0702-4af3-85be-f2a0013937ef

Regards,

Leo

This really goes beyond Office365. This involves the merging of the computing resources of 2 companies.

I would assume, that both independent companies would have

1. Separate Authentication Servers (In this case each have their own AD Servers)
2. Separate IT policies and naming conventions
3. Separate Line of Business Apps (Finance, HR, ERP, Project Mgmt..etc)
4. Separate Security and Backup Systems
5. Separate Networks
6. Separate Productivity tools (Emails, Intranets, etc)

I would start by doing a detailed discovery and mapping out the IT systems of both companies and their dependencies.

Then decide how the merged environment would look. 

The discovery might also reveal how touching one system might affect the rest of the systems in the company.

Getting an experienced vendor/consultant to help you with this might be the way to go.