Since a full Online to Online Quickbooks transfer is not possible via the export/import data function (unless you use a 3rd party app), I would highly recommend following the instructions found here:

https://sellerzen.com/add-or-merge-quickbooks-online-company-to-the-same-email-address

Scroll down to the section titled "Merging or Consolidating Companies Under One Email Address in QuickBooks Online". 

After spending a frustrating afternoon trying to figure this out, I followed those instructions and was able to transfer an old Quickbooks account into a newer account, where I can now manage multiple companies under one log in.

I hope this helps!