The easy way to answer this question is to get yourself a cheap light meter and measure the foot-candles (Fc) underneath both set-ups. You can probably find a light chart on the web somewhere too, that will tell you distance v. Fc, so you can have an optimal set up. I find with HPS, MH, and HID lighting, you typically want it no closer than 18" or so otherwise the plants will burn from the heat.

If temp is a problem with your two lights, I would personally opt for just the one in order to have a stable temperature. With the two lights, you would get more growth due to addl. Fcs, but you lose that extra growth with the heat going above 85F.
cptcannabis Reviewed by cptcannabis on . Less light lower or more higher? Hey ATM i got 2x400watt HPS lights in a smallish tent, i can either have one light on at about 30cm or both on with one at about 40cm and one at about 60-70cm to create an air gap in between as the temp gets too high if i do it any other way, i was wondering if the latter is going to have better results or if im better just using one and saving power. Rating: 5