GT: Nico Loire
Just need to do 'Double the Fun'.
To get Double the Fun all you have to do is defeat the psycho in case 3-3 while playing with a co-op partner. To find a co-op partner use this thread here. To make it easier for your partner, make a save after finishing Case 3-2. Then all you have to do is reload this save and defeat the boss with your partner and you will both get the achievement.
Should be self-explanatory. Just add me.