This achievement is gained by having two players play on the same team during the whole of the Euro 2012 championship - and winning, of course. This is done in the UEFA EURO 2012 mode (offline). You can either play with a friend sat next to you and win the tournament or you can play with a second controller. To make this easier you can change all the sliders of the CPU right down. If you are playing with a second controller, be careful because if the second controller turns off due to inactivity, the achievement becomes unobtainable and you'll have to start the tournament again. Try using a wired controller for the second controller if you have one.