You must have completed the co-op campaign.
You cannot get this achievement on split screen.
You have to have them on your friends list.
You can also create temporary accounts that get 1 month of free Xbox Live Gold and you can get this achievement with a friend.
They need to generate a new Windows Live ID -- http://bit.ly/mRbajB
-- (I went with a generic, DKBOT e-mail address). Sign in with a new account on your Xbox. Attempt to sign-up for Xbox Live Gold, select the "Upgrade Later" option and sign-up under a Silver account. The next screen will offer you a free month of Xbox Live Gold with your new account and accept this offer.
There you go, a free second account and you don't have to find a 48-hour code or attempt to track down a new friend who hasn't started the co-op.