Using any light efficiently isn't about the number of plants; it would relate to the total area of your canopy.. If you have 2 plants with a large canopy, then it would be efficient enough. The intensity of the light just diminishes, of course, the further away you get from the center & bulb itself. DWC can make large plants, quickly, so a 600 Watt light for 2 would give you an almost maximum yield- if there is a good balance between your elements.