Start 'Spec-Ops' in Splitscreen (playing yourself as the 2nd pad is fine)
Go into MILEHIGH JACK and change both players to 'Veteran Difficulty'.
Now 'Change Op' to STAY SHARP.
Complete the easy assault course level, get 17 kills each for another achievement and both this and 50/50 will pop.
Or just run through as player 1 if you only need this achievement.