EDIT: THIS ACHIEVEMENT IS NOW UNOBTAINABLE
I thought I would write this guide for anyone who is concerned about the 'Quitting Out' method with two gold accounts, two xbox's and two copies of the game. I realise that not everyone has two xbox's, but for those who do hope this helps.
To start install the game onto your hard drive for both xboxes, this will make it slightly quicker with loading times. Also both accounts should roughly be around the same rank. When i first attempted this method i couldn't get my two accounts to find each other because my primary accounts rank was too high. If you want to lower one of your accounts ranks, just join a ranked game, and do nothing, eventually your PPH will drop and you will get to bottom rank.
Both accounts need to create a clan. Once both clans are created, ready your clans and have your primary account send a clan match to your second account, CTF or conquest, any map, it doesn't really matter. Once both accounts are in the game, press start and ready up both clans, the game will then begin. Make sure you spawn with your primary account otherwise it won't count, as you have not participated. As soon as you spawn, take your second controller and quit out. You should see with you main account that you have won and the scoreboard will appear. Wait until the vote map screen appears and vote for any map, this will pick the same map as before and load it very quickly. Invite your second account into the game and repeat the whole process.
If your concentrating on the game you can start a game, finish and start another in 2 minute or less depending on how fast you can invite and how quick the game loads etc. I found that this way is just as efficient as boosting the ranked Dammage games, it may even be slightly quicker. This method also allows you do it whenever you want to and for however long you want to.
Hope this clears a few things up for people as when i first heard about this method is what unclear as to what i had to do until i sat down and did it.
Credit goes to Gaz Insane and Jayour for explaining this method to me.