How can I merge 2 invoices to one customer at the time of payment

Thanks for posting here, @arthokart.

I'd be happy to show you how to select multiple invoices when applying a payment to one customer. All you have to do is checkmark the invoices you want to pay. Here's how:

1. In the left menu, click the Sales.
2. Go to the Customers tab.
3. Choose a customer you want to receive payment.
4. Click the New Transaction drop-down button and tap Payment.
5. Enter all the information needed.
6. Put a checkmark beside the invoices you want.
7. Choose Save and close, Save and new, or Save and send.

For more details about receiving invoice payments, see this article: Record invoice payments in QuickBooks Online.

For future tasks, you may find this resource helpful: Undo the Receive Payment of an invoice.

Feel free to visit the Community again if you need further assistance in managing your QuickBooks account. We're always here to help. Have a nice day.