I don't recall a way to pay with 2 cc's, but you can double check on the payment page. Alternatively, you can pay with money from your TravelBank and a cc. This means using your Amex to put funds into your TravelBank and applying them at checkout BEFORE using your Visa as the final method of payment - just make sure you put enough in! If you are unfamiliar with the registry, visit
http://www.united.com/CMS/en-US/prod...tRegistry.aspx for more info.
Or vice versa - might be easier to use visa for TB funds!