Hi @Dave Cameron,
Merging two cloud tenants into one can be a complex and challenging process, depending on the size and configuration of the tenant, the number and type of services involved, and business needs and goals.
You can use the Tenant-to-tenant migration architecture model to understand how to plan for Microsoft 365 tenant-to-tenant migrations and the steps of a migration..
You can also refer to the merge/migration ideas provided in this article：How to migrate mailboxes from one Microsoft 365 or Office 365 organization to another | Microsoft Learn
In this article, you can learn how to migrate mailbox and service settings from one Microsoft 365 or Office 365 tenant to another Microsoft 365 or Office 365 tenant in a business combination scenario.
(Kindly note: If you have more than 500 users to migrate or a large amount of SharePoint data to migrate, it's a good idea to work with a Microsoft solution provider.)
Hope the above information is helpful to you!
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