Lumens decrease by half every foot they travel. Usually smaller bulb sizes have more lumens per watt. The ballasts create heat and two ballasts create a bit more heat then one equal to the watts of the smaller two so they are about equal for effieceny I would say but can not say for sure. The light distribution would be more even with two but the spectrum would be more balanced with one due to a little things called photonreturn. Basically waves can cancel them selves in a way creating a different nanometer but I would not be to concerned with it unless the bulbs are side by side in the same reflecter. So basically I have no idea

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