How do I nerge 2 companies and retain the detail of each company in QB Desktop?

I'll share details about combining company files in QuickBooks Desktop (QBDT), @lensani.

Combining two company data files into one is currently unavailable in QBDT. Each company is created as a separate file and cannot be merged. However, you can manually export one company's data and import it into the company you want to keep using the Intuit Interchange Format (IIF) format. 

Before starting the process, make sure to back up your company file. If changes occur, having a backup copy allows you to restore it to undo the changes.

To export your data, follow these instructions:

1. Sign in to your company file as an admin.
2. From the QuickBooks File menu, select Utilities > Export > Lists to IIF Files.
3. Choose the lists you want to export and hit OK.
4. Browse where you want to save your file and click Save.

Then, complete the following procedures to import them:

1. Go to the File menu, then select Utilities.
2. Choose Import and pick Import IIF.
3. Browse the data, then hit Open and OK to start the process.
4. Follow the on-screen instructions.

For the complete process, check out this article: Export, import, and edit IIF files.

Alternatively, you can look for a third-party app that offers a migration tool or supports the merging service process. You can browse this link to find an app: QuickBooks Desktop App Store or find one outside the QuickBooks program.

Moreover, you can check out this article to learn more about combining reports from multiple company files without limits on how many financial statements you can consolidate: Combine reports from multiple company files.

If you need further assistance with combining QuickBooks data files, feel free to let me know by posting a comment below, and I'll get back to you.

@lensani 

Consider to purchase a 3rd party service to merge your company files.To merge multiple company 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'.

Also, only one company file can have Payroll transactions. If both company files have payroll, the payroll transactions cannot be merged. Other than that, all transactions types can be merged. Any number of files can be merged into a single QuickBooks Desktop file or converted to QuickBooks Online.