Can sub customers who share the same parent be merged?

Hi joliver-takasho-,

You'll have to make sure to remove the sub-customers first from their parent customer by unticking the Is a sub-customer box.

Then, we can edit and merge the customers. Here's how:

1. Go to Sales and select Customers.
2. Select and open the customer profile you don’t want to keep.
3. Select Edit.
4. In the Customer display name field, enter the name of the duplicate customer profile you want to keep. The names need to match exactly.
5. Select Save.
6. When asked if you want to merge the two profiles, select Yes.

That's it! You've merge the sub-customers now in your QBO account. 

You can refer to this link to learn more about: Add and manage customers in QuickBooks Online.

Post again in the Community if you have further concerns. I'll be around to help you.