A description of how to unlock the achievement and a listing of people willing to help others unlock it.
How to Unlock the And They’ll Tell Two Friends Achievement
The achievement is still able to be unlocked, even after the closing of GameSpy’s servers. The way to do so is however, extremely specific, but easily done. To unlock the achievement you’ll need to be in a game that’s started from the lobby with the other player who has the achievement.
This means that if you are in control of your character and then you have that same player who has the achievement join your game you will not unlock it. Both of you need to be in the same lobby screen before the game is launched to unlock it.
Launch from this screen to be triple clear: