It's one account but yeah it can be on multiple devices at a time. You can also have your phone or iPad in a completely different market and if u have internet, you get TV. My parents house is in the Philly Market while my apartment is in the NY market. When I visit my parents the channels I get just switch to the Philly stations.
Considering most cable companies charge you a fee per cable box, which is absurd, it saves you so much money.