I think Spotify getting this feature working wouldn't be trivial as syncing audio perfectly across different Internet devices (labtop, PC, tablet, phone, etc) doesn't seem doable. Also, music licensing issues would probably be an issue too.
Well, everyone with a Spotify licence already has the a licence to play songs on Spotify. Don't get what the problem is. Syncing could probably be done via microphone. One device starts and then the other devices catch on in turn. The syncing would hardly be noticeable at the beginning of the song.
Ah, I smell the sort of thing where the only answer you'll accept, and the only answer I can give is, "Go ahead and try it."
You'll find out why it's hard in a way that no amount of explaining will do, and you'll find reality less amenable to argumentation than comment boxes.
(If you do try it, make sure you've got two different devices, ideally two different types, and be sure to be streaming from somewhere at least 50ms away on the network, ideally with at least a few ms in variance between requests. Though odds are pretty good that even trying to stream the same song to two identical devices over a local Wifi network would only work most of the time, you'd find...)
In fact, things that are a great deal more complicated can also be done, such as really fantastic real-time echo cancellation or feedback prevention in a large room with many speakers and many microphones. It just isn't as easy as you would naively think; everything that just leaps to mind (especially if you aren't particularly knowledgeable about signal processing) don't work. You'll pay big bucks for the system that can do that, and there's a reason for that.
Well, Spotify caches songs, so the streaming part is done for you. The problem is constrained to synchronizing playback. Yes, that might be really hard, but you're inventing problems.
I think Spotify getting this feature working wouldn't be trivial as syncing audio perfectly across different Internet devices (labtop, PC, tablet, phone, etc) doesn't seem doable. Also, music licensing issues would probably be an issue too.