Which says:

"US Airways makes it easy for you to earn extra Preferred miles to help you reach status. Members who sign up for a US Airways Premier World MasterCard or Signature Visa card and charge more than $25,000 in net purchases during the calendar year enjoy the exclusive benefit of having 10,000 base miles converted to Preferred miles. Savvy travelers know they can sign up for both cards and earn 20,000 Preferred miles this way. US Airways is the only airline that offers this benefit."

I have not previously seen this explicitly stated. That makes it pretty clear. ^