You are probably seeing them now since more people have unlocked the dual wield attachment after the double weapon XP.
Not sure about youtube people but dual kap40s are a great secondary for a sniper class (they are built into the default class). They are essentially the akimbo Skorpians from BO1 with a slightly smaller magazine but faster ROF. It's no surpirse they are popular.
The B23Rs are just as good. Even more so since they are easier to control. With that said, I hardly ever see them. Usually it is just me running them with someone picking them up off my dead body after I die due to the game spawning a third guy right up my rear. I use them since they are easier to spary from the hip when I miss a shot or get the unfortunate hit marker with the Ballista.
"Cover your receptors Perceptor." Driver picks the music. Shotgun shuts his cakehole.