You can call in, ask about doing it online, when told you cannot, see if they'll do it for you without the booking fee.
Also, I know UA has gift certificates.
http://www.united.com/CMS/en-US/prod...tificates.aspx. You could buy it using your pre-paid or credit card.
When you pay for your ticket, the "redeem gift certificate" section is separate from the "CC payment" section so you cause use the Cert then pay the remainder with the CC.
Also, I think you can pay via paypal. Paypal may have a method for using two cards.