Resistance: Fall of Man had a pretty good separate aim system for the Reapers (dual-wielded alien SMGs), where the two weapons fired on the primary and secondary fire buttons.
The weapons individually locked onto targets, separate or the same one. It's easy to use, intuitive, and not considerably worse than single weapons, and yet FEELS like dual-wielding (unlike dual wielding in, oh, let's say... Halo. Which if you removed the graphics of the weapons would feel pretty much exactly like single wielding).