slinger legionaries actually deployed may well have operated in pairs a bit like skirmishers of later periods. One observing and shooting (scutum slung or on the ground) and one protecting with his shield (or both shields?) and with both soldiers pila.
I am doubtful about skirmishers really deployed by twos at a large distance from others, before it became really useful because of black powder musket fire (from the mid-18th C. onwards). It needs a lot of special training. In previous times I would rather imagine them in a loose order. Also a slinger cannot have much protection from the shields (as a crossbowman would have) as he must do large gestures to use his sling. But I’m not saying you couldn’t send two guys ahead, if the centurion wants to; one watching around and the other shooting.