I will refer to an "attack" and an "action" button. I am calling it these instead of something more specific like shoot or melee because they change jobs. The attack button refers to shooting and melee while the action refers to taking cover and marking executions.
At the very start of the game you are ordered to get a gun. Walk forward, creeping up to an enemy, and tap the attack button while it is displaying a fist. Once you have done so, you will have the option to mark and execute your next target. When you arrive at the second target, the action button should turn into a cross hair and mark the enemies with red dots above their heads. At that point, hit the attack button. Since there are two enemies at the area of the second target it should mark and execute them both.