Tyson Green: Dual wielding is such a natural extension of how we wanted weapons to be used that it wasn't a major change. The weapons are meant to complement one another, and dual wielding simply allows that to occur without the overhead of switching weapons. It did emphasize the individual weapon roles a bit more (for example, the Plasma Pistol is now almost exclusively about taking down shields, and the updated Pistol is mostly effective only at short range headshots), but it didn't force a revolution.
As for concessions, because you're trading your ability to throw a useful and potent grenade by picking up a second weapon, we didn't need to badly nerf them. The largest concession made is that dual wielded weapons are less accurate and end up doing slightly less damage, but only when fired simultaneously. If you stagger your fire, they behave exactly like regular weapons.

Shotgun!
GameSpy: As far as melee attacks with the weapons goes, do they all do the same amount of damage?
Tyson Green: Some of the weapons do slightly more damagethe Plasma Sword and Brute Shot because they are made for doing melee damage (the Brute Shot via the large blade on its stock), and the Rocket Launcher also does a bit more damage when you smash it down over someone's head.GameSpy: Have the grenades changed at all from the last game?
Tyson Green: There are some very minor changes in the way they bounce and behave, and it is now possible (albeit extremely difficult) to snipe and detonate them midair, but they are pretty true to the original aside from that. Frag grenades still bounce better than plasma grenades, and plasma grenades still stick to hapless victims.GameSpy: How was the decision made to allow rockets to lock-on to targets?
Tyson Green: Emphasizing the role of the rocket launcher as a vehicle killer, particular as the counter measure to a Banshee, made it a pretty easy decision to make. The final lock-on mechanism is the result of many iterations of design, implementation, and playtest.