The idea of an actual third person involved never occurred to me. Neither did the idea of my left hand engaging my right one, or vice versa
. Just wondered, in case of the imperative, what person's hand I'm addressing: my own or the person's I'm greeting. Both are technically third person singular.
¡Vengan esos cinco! can be readily translated into Russian, by the way. Sounds just as informal.
Dame los cinco, literally.