Party of Three Achievement
Party of Three25
Find the hidden companion cube in co-op test chamber
This can be gained in Course 4 – Excursion Funnels Chamber 9 and can be gained in either split-screen or over XBL.
This is pretty much at the end of the 9th chamber. Once you make your way through the last checkpoint gate use the Excursion Funnel to get across the first gap and make sure your partner blocks the turrets. Take the funnel to the end and look down to your right for a place to put another portal to carry on your journey.
Place the portal and fall. As you enter the new area, you'll see a walkway to the left and a office type building to the right and then the gap you're going through inbetween them. Just before you reach the office, look left and re-direct the funnel so that you're now heading right along it.
Just stay in the funnel and keep looking along the same wall as the office. A wall will open which will reveal the cube and the achievement will unlock.