Welcome to the Community, 1accounts-arnhem!

You'll have to remove the sub-customers linked to each parent customer before merging them in QuickBooks Online. Here's how to remove sub-customer from the list:

1. Click on the Sales tab.
2. Choose Customers.
3. Locate the sub-customer name.
4. Click on Edit.
5. Untick box for Is a sub-customer.
6. Click on Save.
7. Repeat the same process for other sub-customers.

Once done, you can merge the two parent customer after.

Furthermore, here's how you can export the customers via excel in QBO.

1. Click on the Sales tab.
2. Select Customers.
3. Click on the Export Icon just right above ACTION column.
4. The file will be saved under your Downloads folder and ready for use or editing.

For reference you can check this link to learn more about: Merge duplicate accounts, customers, and suppliers in QuickBooks Online.

Post again in the Community if you have other QuickBooks concerns.