Robert Mason states that they used to formate at night by using the instrument lights of the other helicopter, with overlapping rotor discs
You can never really know what folks do get up to, but as a driver of the type engaged in the same shooting match find it extremely difficult to believe.
Interesting to see the local Ospreys flying night landings in V formation, no lights, and request to the tower to turn OFF the runway/taxi way lights prior to joining the circuit.