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.