Kinect Adventures

This secret achievement is called:

That's What I Call Teamwork! for 50 gamerscore.

I have confirmed that you can get this by playing coop on all the free play levels and it took me roughly about 2 hours to complete.

You just have to complete the level coop, you do not have to get a platinum or gold. You most likely will obtain a medal of some kind in coop anyway though.

I could not see anyway to track which ones you have already done so make sure you either write them down or start at the very beginning and work your way through everyone of them.

Credit to FreedMania for confirming you can do all these online with another player also. Details in the comments section of this solution.

If you find this solution useful please don't forget to vote positve. Thank you and good luck!

This is possible to do by yourself. I heard about the pillow trick, but I could never get it to work. You'll have to trick the kinect sensor into thinking there is another person there.

I used a bar stool and a candle stick on top of it with a blanket thrown over top. You can use just about anything. Stack things up on a chair. Use mic stand...etc

This works fine for all levels except reflex ridge. On reflex ridge you have to start by putting your arms out and pulling backwards and is obviously not easy to do with a bar stool. So for these levels start them by yourself and right after you start move your object into view by ducking down behind it. The kinect will think that is you. Then move to the side and it will connect you as the second player.

You do not need to have two players from start to finish. I finished every level with two players just to be sure. The kinect sometimes loses the object or you, this does not matter because you can join in the middle of the level and it will still count. Also does not have to be done in one sitting.

As WolfVelocity suggests, this can be done in around 2 minutes from scratch if you have a partner with you. if you've already made some, progress, it will take less time. If you want to also get the Platinom Nom Nom achievement, I'd recommend trying to get platinum medals on all of the freeplay levels together with your partner, so that once you're done obtaining this achievement, you won't have that many to improve on. If you can get platinum medals on all activities with your partner, then you'll obtain this achievement and Platinom Nom Nom, requiring no later score improvement. Playing together with a partner makes it much easier to get platinum medals in the more difficult Space Pop, Rally Ball, and 20,000 Leaks where you need to cover different parts of the screen quickly.