Actually, the reason the bottom barrel on an O/U normally fires first is because it is the barrel that is closest to the pivot point(hinge pin) and puts less stress on the action. Firing one shot is a much more common occurrence than doubles, so the stress doesn't even out over all. You can see this theory with O/U combination guns, that usually position the rifle barrel on the bottom. Of course there are exceptions, one being "low pressure" rifle cartridges( such as rimfire, 8x58R S&S, etc) and extremely strong actions( like some Brno models). As to whether double or single triggers are faster, I shoot double triggers and am never aware of which trigger I'm pulling, except when I want to use the tightest choke first.
Mike
