Take Akimbo G18s (I also confirmed it does this with Desert Eagle, so it probably works with Akimbo pistols and machine pistols, it doesn't happen with the Akimbo 1887's).

Take your Akimbo G18s and hit the stab button. Your right hand gun will have a Throwing Knife sticking out from the end of it.

This can only be seen in split screen (by another player) or by another player in MP (Presumably, I didn't try it online). You will never see your own right hand during the stab script.

On a side note, you can throw Throwing Knives into your Teammates as long as Friendly Fire is off. They act like a storage spot for knives. If that player has Throwing Knives equipped, he can use the knives that are stuck in his body after throwing his own.