There are plenty of opportunities to get this during the Single Player. One of the easier levels to do this is during Mission 4 - "Taking Down the Rigs." There are plenty of enemies in tight areas. The grenade has to kill both of them, no other explosive things (explosive barrels) can be set off to kill them.
This can be done in the very first chapter. After you crawl and kill the enemy, you will take a right and be told to hide in the bushes as enemies run by. After they pass, go out of the bushes and hit to throw a grenade at them.