To split a payment between two credit cards, follow these steps:
This is a workaround.
Remember, IOUs are deposited to the business owner's merchant account.
Go to the Checkout screen.
Select a customer.
Add services, products, etc., to the cart.
Enter an amount in IOU.
This will be the payment amount for the second credit card.
Charge the first card. To learn how to charge customer cards, see here.
The Add IOU pop-up will display as you charge the card. Enter a comment, then click Yes.
After checking out the partial payment, go back to the Checkout screen and select the customer again.
The IOU will display in the cart, and you can charge the IOU amount using the second credit card.