Two for One

Two for One is a Halo 3 multiplayer achievement. It is awarded for killing two enemies with one Spartan Laser shot in a ranked free for all playlist (i.e. Lone Wolves). Due to the difficulty in earning this particular Achievement, people have been working together to let others earn this Achievement. The Achievement awards the player 5 Gamerscore.

It is represented by a white circle with a red star. On top, two skulls and a Spartan Laser silhouette.

Method 1
Another way to get this achievement is to also use the Spartan Laser on King of the Hill gametypes, where everyone will be grouped inside the hill and not able to pay attention to a laser charging up in the background.

Method 2
One more way is plain and simple and is to have the Spartan Laser on maps that contain Type-32 Ghosts, and all you would need to do is charge the laser and get the right moment for when someone boards the Type-32 Ghost, using the Laser to destroy the Type-32 Ghost will result in the driver and the boarder being "Lasered". Although this can be a quite difficult and time-consuming tactic.

Method 3
Another way is, to ask somebody to help you with the achievement during a match. Simple (then again, potential interference from the players who are not there to help you can still make it not so simple).

Method 4
In the Double EXP Weekend playlist Legendary Brawl, occasionally there may be two players on a Warthog. When you spot it, be quick to destroy them with a Spartan Laser, as you're probably not the only one that doesn't have the achievement.

Note: This is only obtainable on Avalanche, Standoff and Rat's Nest, as Ghost Town does not have a Warthog (but it has several Mongooses) and the Sandtrap variant in the playlist is "FFA Modified".

Method 5
On a standard Slayer match in Lone Wolves, simply wait for two people to start fighting and follow them whilst charging your laser. When the two go to beat each other down, as people often do, a well-timed shot should be able to hit both of them regardless of what angle you have on them. This method is actually not all that hard, but requires you to be in the right place at the right time.

Method 6
Sometimes in Oddball or King of the Hill, players will team up to hunt down the objective. This is a good time because you can charge up the laser and hit two players.

Method 7
A good way to get this achievement is to find two people that are away from the game and line them up and fire your laser, or you can find one person away and get a friend to stand in front of them for you.

Luck
Sometimes, this achievement is scored by randomized luck and fortune when you try to kill one person and another comes in the way of the other for a close-combat duel. Of course, this is dependent on nothing, but a straight-shot from the Spartan Laser and a great deal of luck. There are also many players in the Legendary Brawl playlist that help each other get this achievement.

Trivia

 * When getting this Achievement, remember it doesn't have to go through both of them, the shot just has to kill two people.
 * Due to the difficulty of getting this achievement, many players refer it as one of the most difficult achievements to obtain among the original 40 achievements.