If I have a Nexus Player, and connect to it with more than one phone (Using Android TV Remote Control), is it possible to distinguish between each controlling unit in the code?
(It would make for a cool way to do multi-player games without the need for expensive game pads - friends can just use their phones)
Yes, you can do this. Each input device is given a unique identifier when connecting to an Android device (including the Nexus Player). Refer to the documentation here. I believe there are already multiplayer games on the Nexus Player which accomplish this (seen here).