With proper ventilation you can keep your lights inches from your plants, it's all dependent on your temps. I personally disagree with the 2x2, 3x3, 4x4 rule of thumb I'ld say you can easily go 400w 3x3, 600w 4x4, and 1000w 5x5 without problems of light loss. So with that your 2 600's can cover a 8x4 area rather well. 8x8 just doesnt even make geometric sense, unless your lights somehow send out a wide rectangular footprint.