Where is the hidden weighted companion cube when playing Portal 2 in co-op? (Party of three achievement)

Solution 1:

It's at the end of the 9th Chamber of Course 4: Excursion Funnels.

Remember the part where you have to create a light bridge that crosses in front of the turrets on the conveyor belt, and they all just hit it one after the other and die? Normally you'd have to move that light bridge slightly to the left so that you can continue with the level.

Instead of moving the light bridge, place the exit portal of your excursion funnel beside it - so that you start moving backwards into the unexplored depths of the level. You'll pass by the cube, which you'll be able to see through the light bridge. Then you will die.

The achievement should pop here automatically. Here's a video of the process.