The difference is light penetration. The 400w light is only good for up to 14" after that the lumens diminish fast.

Your 1000w light can penetrate up to 18" well and 24" is not bad but a little far. If you can install glass as a heat shield in your 1000w light that will help allot with heat issues. An air-cooled hood where you bring in air from outside the grow room. Push it though the hood and back out the other side of the room will cool your bulbs.

If you have room (not sure if you do) stager the lights on each side of the plant one closer then the other and hang them as top/side light.
Zandor Reviewed by Zandor on . waisting light? Well it goes like this,i have grown out doors for a number of years now .I am on my 4th indoor grow and dont have alot of room these days about 4.5 foot x less than 2 foot wide by about 8 foot tall have ducting in and out. I am runing 1 x 400 watt son-t (think it is called) & 1000watt MS 1000 supergrow for flowering.I have the 400 watt on for 12/12 & the 1000 watt on for 7 hrs of there day these hang side by side. Things get a bit warm but stay ok.Into the third week of flower and Rating: 5