I'll share some information to help you manage your company file, NBB2022.
You can have multiple company files in QuickBooks Desktop. The Pro Advisor is correct, you'll have to generate a new company file if you want to combine the two company file your client have into one main file.
Just to be sure, did you use the feature to create a new file from existing ones? If yes, the software copies your preference, sales tax items, memorized report, and chart of account to the new file. However, it doesn't transfer credit card and bank accounts. It also doesn't copy sensitive information like payroll or Employer Identification Number (EIN).
It would be a long process for you combining data from different files. With this, it's best to re-enter the new invoices and bills into the old files. This ensure you'll have all the transactions when you're ready to migrate to QuickBooks Online.
Before migrating your data, I recommend reading this article to learn how your books were moved: Learn how features and data move from QuickBooks Desktop to QuickBooks Online.
Additionally, I've added this resource to walk your through the process of transferring your QuickBooks Desktop file to QuickBooks Online: Move your QuickBooks Desktop file to QuickBooks Online.
If you have further questions about managing your company file or QuickBooks data, let us know. We're always available to help you.
