I cant answer specificaly for the AR platform,or even recent set triggers, but when I think of a 2 stage trigger I think of a set trigger, whereby you have two stages of trigger pull. The last gun I shot with a set trigger was a CZ in .270, to set the trigger you pushed forward on the trigger whereupon it engaged its second stage with a very light pull( probably around 6-8 ozs or less) . We all assume a light and crisp trigger pull will increase accuracy, and it does in a single stage trigger, but with a set trigger, at least with ones from the 1970's and earlier, it increases lock time. Benchrest shooters and the like have long strived to minimize lock time therefore reducing the amount of error in each shot. I dont own any rifles with a set trigger and probably never will, but not because I think they will degrade accuracy. Keep in mind this is mainly pertaining to bench and competition shooting. In a hunting situation(which is what a majority of my guns are used for) I would rather have a well tuned single stage trigger so I'm not fumbling around trying to get a set trigger engaged, but that just personal preference.