Cross tenant Migration of user data and emails office 365 to office 365

[RuaJefwa]

I am currently working on a cross tenant Migration and the downside of the Migration tool in exchange is that once the email data is moved to the target tenant the source email address is not accessible while our requirements is that we need to maintain both email addresses for about two weeks or so before we terminate the source email address. Could someone suggest a solution on this or a 3rd party tool where necessary and is this possible.

[#2]

Hello!
What do you mean with "maintain both email addresses"?
Once the mailboxes are migrated, if someone sends a message to the source mailbox it will be redirected to the target one.
What do you need exactly?

[#3]

No tools will allow this without performing additional work. You can do one of two things. The first is to set up forwarders in the source tenant to forward mail to users in the target tenant (either on the mailbox or as a transport rule). This option does not allow the migrated users to have a from/reply-to address of that domain though. Be aware that forwarding is disabled in Exchange Online by default so you'll need to allow this.
The other option is to remove the domain from the source tenant, add it to the target tenant and configure mail routing for that domain to go to the target tenant. There is some downtime with this as you need to first remove the domain, then add it to the target tenant, then add the required smtp proxy addresses to your users in the target, then configure mail routing to the target tenant. This can take some time depending on the size of the tenants, and mail will need to be queued somewhere during this period.
If the source domain is not needed after two weeks I'd recommend the former rather than the latter.

[#4]

I need users to be able to access both email addresses in the source and target tenant after the Migration

[#5]

Yes I did follow the Microsoft guide. Once the migration is done the source email is deleted from the source tenant and you cannot access it you get the 500 error for the mailbox it is actullay converted to a mail user account

[#6]

@RuaJefwa If you need that users can use both email addresses to send mail, that's not possible after the cutover. In those scenarios, usually we sync the migration batch but we let it synced, ( without complete ), until it's required. 

Other option is to transfer the domain to the new tenant, add it as alias to the users, and enable in the tentat the option to send from aliases. 

[#7]

What tool are you using for the migration of mailboxes from one-tenant to another? I've used a number over the years, and none of them convert the source into a mail user and remove the mailbox. Mail user accounts can have a TargetAddress, which forwards mail to another email address, but there's more details in my original reply