On My Mark
From Halopedia, the Halo wiki
This achievement can be earned as early as the beginning of the level. After entering the first room and stealthily killing the two Grunts in it, two players should climb into the vent shaft above where the Grunt on the left normally spawns. One player should move down the shaft, while the other should go to the right. Both exiting ends of the vent shaft have an Elite each. When the players reach their end of the vent shafts, they should count from 5 to 1, drop down onto the Elites on 1, and then assassinate them. If the Elites are successfully assassinated at the same time, the achievement is then unlocked for all players playing in the mission.
If the Covenant troops were alerted, it becomes much harder to get the achievement, thus requiring the player to restart the mission from the beginning and stealthily kill the two Grunts in the first room.