If you're desperate to get this achievement by yourself, there is a way. When you start a song in any mode and it gets to the "Who's playing?" screen, you have to get a little creative. Walk into view of the kinect with something attached to your front. Vacuum cleaner, barstool, anything that you can quickly fool the kinect into thinking is you. When you see the object you're holding turn blue like yourself, you have to set it down, crouch behind it and hit the xbox guide button to pause your game. After you have done this, you have to step out of view of the kinect, press the xbox guide button again to effectively unpause the game, and then step back in front of the kinect as if you're a new person. Raise your arm up into the air and the the game should recognize you as a second person. You must do all of this before the "Who's playing?" screen goes away. Finish out the song and the achievement is yours!