How do I move a company with a single user into another company?

In QuickBooks Online (QBO), you cannot directly move one company file into another, Lindsay.

Please note that in QBO, each company is a separate entity with its own unique data and subscription. Changing the email addresses to be the same also won't allow you to switch between them as if they were a single account. Each company file is tied to a specific subscription and login, even if the email is the same. The inability to switch between them likely means you are logged into two different accounts rather than having a single account with access to multiple companies.

As a workaround, to move data between companies, you can export certain lists like customers, vendors, and the chart of accounts, and then import them into the new company file. However, this method will not transfer transactional data like invoices, bills, or bank transactions.

You can also check out this article about detailed information about exporting and importing lists to QuickBooks Online: Move your lists to QuickBooks Online: Move your lists to QuickBooks Online.

Come back to this post if you have other questions or concerns.

@Lindsay-Nagle 

You can purchase a 3rd party service to migrate your data.