@Al Jones 

To combine multiple QB Desktop company data files, the chart of accounts in both files cannot have the same account name with different types. For example, if one company file has an account with name 'Accounts Receivable' of type AccountsReceivable, the second company cannot have an account name with the name 'Accounts Receivable' of another type. To resolve this issue, rename one of the accounts to 'Accounts Receivable1'. 

Please note, during the conversion process from QB Online to QB Desktop, and QB Desktop to QB Online again, you will find some data types missing (e.g. non-posting transactions, attachments, jobs, classes, custom fields). You will have to spend time doing manual work to migrate them.