Can't you have two people signed in to the same session? If not, just use the same card.
Nope. If you try to log on from two computers, the second login will kick the first login offline. So you cannot have 2 characters from the same account online at the same time.
There also isn't anything like a master account. At least not yet. If you don't want to have your CC info on two separate accounts, the only option you have (at the moment) is to pay for the second account with timecards.