Just the same has what TheFourthHunter above said if you have more then one controller. Also I did this on my Xboxlive account and with another account but am pretty sure you can get the achievement on one account signed in has a guest. And you can get this in custom match with one of your friends has well of course. And through online means has well but this method is the easiest and quicker in my opinion.
Select custom games Halo 2 then select Halo Anniversary then select the map Shrine. Then go over and select Multi-flag start the game up. Make sure everyone is on opposite teams of course. Go over to the Red base if your on the Blue team and vice versus. Then look up shoot the redish orange light run in and grab the flag and come out through the waterfall and the achievement should pop up. You can unlock this achievement in matchmaking of course..
*NOTE for some reason the achievement didn't pop up on the video but I did unlock it tho.