You can get a second (and third and fourth) by prepaying the reduced rate of $83.88 (the additional receiver rate). After ordering the first, you should be able to log in with the username and password you chose and it should automatically sense that you have another subscription. If it doesn't, you can call customer service and they should adjust it for you.
I signed on with my user ID and it showed the option for the $6.99 mo for extra receiver and would show up when ordered. Well it didn't and charged me for the year. I have to wait till I get both receivers and then call CS to get an adjustment on my CC.