Unless the receiver is connected by a phone line, directv cannot detect its use. I think that this is how so many people get away with using bootleg access cards. The phone line is needed for purchasing pay-per-view movies and not for getting service. If you have a valid card, you can get service with no phone line attached.
Keep in mind that if you are caught using a card that has not been authorized by Directv that you can be fine and criminal charges can be persued.
Let me know if you have more questions.
I don't personally see why you could not do this but you should check with directv just to be sure.
If you decide to do it without asking them, be sure that the alternate receiver is not attached to a phone line or your access card might get zapped!