i found this video, but its mad blurry so i cant really see how he does it very well
[ame=http://www.youtube.com/watch?v=sQOYBH_Pg-I]YouTube - Gears Of War Achievement Glitch[/ame]
ive got an account with 1250 on some of those ach's were spg from 360gs about 2 years ago,its simlar to the gow2 spg but use 2 pads.
select the main profile with the 2nd pad but press A on the 1st pad that you signed in with dummy profile.
sorry if thats hard to follow it was ages ago when i did it