Yeild

yeah it could, theoretically if you have enough space they will yield more but the extra hps will make the temps raise and the humidity will drop down so it could create a situation to manage for you. If your grow room is less than 1 m2 then it's useless ass 2x400 is already too much if you wanna get the optimal environment parameters . in a 1.5x1.5m you can drop 2x 400w or 2x600 with a ballast with a dimmer, without dimmer it's tricky

Hi I think rotating the plants is always a good thing if you 2want things to be somewhat even. Especially if you have different lights in there(different brands/models or styles). I had 2 different leds at 1 point and 1 was so much better with flowering and i didn't rotate them and the 2 plants turned out so drastically different with end flowers.