This is an extremely easy achievment, one of the first you will get while making progress through the campaign. Through the hordes of MW2 enemys theres bound to be a lot of situations when 2 enemies are one behind the other. Yes as "Chizzle e4" said you can shoot people head on while they are on a snowmobile in the Cliffhanger level. Notice in the achievment description it mentions you can obtain it during playing special ops. I personally have got many 2 for ones, collaterals, double kills (whatever you want to call it) on the first sniper mission in Special Ops where you are on a building with troops running towards you. Note, you can't get it with a grenade launcher, only with a gun. Guns can shred the AI to peices so if there happens to be the instance in which someone is standing behind another enemy all guns have enough damage power to penetrate through the first to kill the second (Especially LMGs).