I believe the dual shoulders mostly affects the weapons. I notice with the auto-guns, if a set of arms only has one shoulder slot, it'll shoot one bullet at a time. If it says it supports both arms, it shoots 2 bullts at a time. Pretty much splits your ammo between the arms and lets you shoot twice as much in one go. I WANT to say it pretty much doubles the effects of your shoulder units but I'm not sure (ie subcomputers giving your twice the stats for almost instant lock on and whatever) but I'm not sure.