Gang wars work wonders for hitman levels. Desert eagle should be dual wielded. But I guess it's too powerful. Melee weapons have no skill levels. I have reached hitman on all available weapons
I've done also shooting ranges well 12 out of 12 and have hitman for everything, however RPG, sniper rifle and the knife dont get levels. I got most of mine in gang wars.
The following weapons can be dual wielded:
9mm
Sawn off Shotgun
M4


?
Tec9
SMG


Micro SMG
However i have never had the combat shotgun, so unsure. Shooting with headshots, lights and car tyres also give good rep's if pulling off in one shot each.
The sheer kickback of an M4 without a second arm to support it would blow your shoulder off. Same with an SMG.
Hitman levels on the rest of the weapons just improve accuracy, damage, and movement while aiming with the weapon.