When your customers want to pay an invoice separately, you can easily perform this in the following way:
Choose the opened invoice in POS at Menu > Invoices > Open invoices and click on the blue "Pay *** products" button in the bottom.
Click on "Pay separately", generally placed on the top-right side.
3. Choose the items, which should be paid separately by tipping on the square buttons on the right, and click on "Pay".
4. Continue then with the second part of the invoice.
For your information: You can split the payment many times, until no items are present in the cart.