Let's get this resolved, @AlexisR. I've got the steps to help you achieve your goal of converting a customer to a customer's project in QuickBooks Online (QBO).

You can accomplish this by temporarily creating a new dummy customer, making Customer A and Customer B sub-customers and then converting them into a project. First, create a dummy customer with a different name than Customer A.

Once you have a dummy customer, let's have it as your parent customer that you will rename to Customer A later on. Follow the steps below to proceed making Customer A and Customer B sub-customers:

1. Go to Sales, then select Customers.
2. Pick Customer A and Customer B and click Edit.
3. Select the Is a sub-customer checkbox.
4. Choose the dummy customer as a parent account. This is the account that's connected to the project.
5. Tap the Bill parent customer checkbox.
   

After that, you can now convert sub-customers to a project. Let me show you how:

1. Go to Sales, then select Customers.
2. Click the New project ▼ dropdown and then Convert from sub-customer.
3. Pick Convert to complete the process.
    

Finally, you can edit the projects' names and the customer name accordingly.

If you have further questions about handling projects or customers in QBO, please let me know by commenting below. I'm always glad to help. Have a good day!