It's possible to allow customers to checkout with multiple credit cards, or payment methods on the same order. This is something the AmeriCommerce online stores system refers to as "Split Payments".
To enable this, you will want to go to Settings > Orders > General, and look for an option labled "Allow Customer To Split Payment On Checkout". Check this option on to enable the feature.
After enabling this option, you will see some additional options on the checkout pages under the payment methods. The Reward points and gift certificate sections will let you apply a specific amount of points or a specified gift certificate amount. The Credit card section will allow you to add multiple credit cards, and choose how much you would like to charge each credit card.