So I had Spotify playing on one machine, went upstairs and opened up the laptop where Spotify was already open and clicked on a track to play it. Thought maybe I'd get away with two instances of Spotify running from the same IP, but no. However, it's slick the way it allows itself to be open on two machines at the same time without throwing up an alert box and logging the user out. What happens is that the second you start to play a track on one machine, then the track playing on the other machine pauses instantly, awaiting your return. So clever!