Shopify POS supports split tenders using different payment methods. Simply change the first payment method amount to what the customer wants to pay (tap the amount to edit). If the amount for a current payment type is less than the total amount of the order, you will have the ability to add a new payment type afterwords. This works with credit card transactions made through our card reader too.
More details are provided in our docs under "Customer requires multiple payment methods": http://docs.shopify.com/pos/operating-pos/checkout-receipts/choose-payment-methods
Hope this helps!