It's not about strength, its about co-ordination and dexterity. Sure, most people could hold, and even fire two pistols for a while. Hell, your average infantryman could probably do the same with a polymer and titanium weapon (which the SMG is probably constructed of). But accurately? No. A normal human being doesn't have the dexterity to compensate for recoil from two weapons at once- hence sticking with one. Chief has an AI running alongside his brain to let him use two at once, and the dexterity of someone who's been learning how to fight since the age of six- ODST's may be the next best thing, but they wouldn't be able to accurately fire two 50 caliber pistols at once, let alone a pair of front heavy sub machine guns.