Forum Discussion
Joining a bigger O365 without loosing administrative control
Since the end goal is to inevitably have all the users in the same tenant, it's best to just go with it. You can always assigns a limited role to the IT staff from Company A, with just enough rights to create mailboxes and perform whatever other tasks are necessary. It will be much easier/cheaper solution.
That said, there are some third-party products that can give you cross-tenant admin permissions, but of course they come at a price.
Addendum: The reason they want to integrate into the O365 environment is partly a wish from companyB, but mainly for the Cooperation capabilities. The other option for the latter would be https://docs.microsoft.com/en-us/office365/enterprise/office-365-inter-tenant-collaboration?redirectSourcePath=%252fen-us%252farticle%252fOffice-365-inter-tenant-collaboration-eb45fd8b-1d5d-4b0c-9c5a-479dbb176e7d