You can use multiple credit cards to pay for your order by making multiple credit purchases.  Suppose for instance that your order costs $2000, and you want to put $500 on Card A and $1500 on Card B.  First, purchase $500 of user credit using Card A.  Next, purchase $1500 of user credit using Card B.  Your user credit balance will total $2000, and you can use that credit to pay for your order.