SONIC FREE RIDERS

This can be done by one person. Just change the player by step out of the kinect-field and re-connect. You will be identify as player 2. Then replay the step as often as necessary.

Miles' method did not work for me for a while - I was always recognised as player 1 no matter how many times kinect lost sight of me.
I got this achievement eventually by putting a coat around a chair and laying some jeans on it. I also put a box in the coat with some 3D glasses to act as a head, along with some slippers as feet.

By crouching behind the chair and standing up again kinect was tricked into letting me pick second player eventually. Racing was extremely temperamental - sometimes the chair's character would go left constantly, other times it would stop dead. As long as it keeps moving it will finish eventually. If it stops; I managed to rotate the chair slightly so the game thinks the chair is standing sideways and player 1 starts moving. (NB maybe using Vector or Eggman would work better here due to using a Bike instead of a board?)

After a couple of races with the chair as player 1 it was getting dark outside and kinect stopped recognising the chair(no infra-red maybe? I dont know why exactly since kinect still sees me in the dark). I left the chair in the same position anyway but I was able to walk up close to kinect and walk back to swap to player two after picking the character for player one - essentially the same as in Miles' method.

I think with it being dark, KinectID failed to recognise me as well and this is what eventually allowed Miles' method to start working for me.

If you try this, remember that everything is temperamental; from the character select, to the calibration, to the race. I had fun getting the better of Kinect but to attempt this requires patients. Failing that, try and get a friend to help you.