I'd say at least Pistols and SMG's should be dualies.
Shotguns, Grenade Launcher, Rocket Launcher, Rifles, Mines, Grenades, RCP90 and Laser should remain single.
Dualies could make the pistols and SMG'S a little more appealing.
Ofc. only if you know how to handle them.
If you're spamming you'll only waste twice as much ammo, but if you use them the right way, you'll have nearly twice as long ammo before you need to reload.