If you're an unfortunate b****** like me and all your friends for some reason hate Gears, there is an easy way to get all the Dom chapter completion achievements and all you need are 2 controllers and 2 profiles (Your one + a secondary).
Simply start the game up with a secondary profile on your hard-drive/mem card and start a level on single player. Play through as normal, and then literally just before you finish the level, pause, turn on your secondary controller and sign into it with your account (The one you want the Dom achievement for)
This will automatically create a split screen game, and you can carry on from where you paused it. Simply unpause, finish the last second or two of the level and bam! =)