Not an expert, but you can spread more light out with 2 600's you would also have two bright spots instead of one.. from what I read 600 are the best lights to have 2 of.. I qoute "The 600-watt bulb has the highest lumen-per-watt conversion (150 LPW) and can be placed closer to the canopy of the garden than 1000 or 1100-watt bulbs", so two would be ideal.. I wish I had two.. I am thinking of getting a 600 watt after my first grow and put it with a 400 watt..