You can get the Two Corpses In One Grave Achievement in chapter 3 (ONI: Sword Base) at the beginning of the level after you get through the enemies and down into the bunker where the hammer of dawn target locator is. Grab it and head outside to the right of the two rocks. You will see two wraiths down below, once they get in between the rocks hit them with the target locator. Make sure they are close enough so the air strike hits them both. If you don’t get it reload immediately before the checkpoint hits and try again.

Other Halo Reach Achievements:

Banshees, Fast And Low Achievement

If They Came to Hear Me Beg Achievement