The CloudMigration Office 365 Backup Tool is a robust solution for making Office 365 tenant migrations easy. It supports the transfer of the domain, but it also takes care of securely getting user mailboxes, shared mailboxes, and other resources from one tenant to another.

Steps to merge Office 365 tenants:
1. Download and install the CloudMigration Office 365 Backup Tool on your computer.
2. Login with administrator credentials for the source tenant.
3. Select mailboxes you would like to back up and provide a secure storage location.
4. Remove the domain from the source tenant.
5. Add and verify the domain in the target tenant.
6. Log into the target tenant using the tool.
7. Select the backed-up file to restore mailboxes back to the target domain.
8. Validate that the mailboxes are intact and verify the DNS records for the domain.

Know More: https://www.cloudemailmigration.com/blog/merge-two-office-365-tenants/