12/12 from start to finish

The larger clamp lights, I think 10 1/2 inches, can accomodate a y splitter and 2 bulbs...

the 400w alone might be barely enough to flower 6 plants. but getting the bulbs i posted a link for would benefit you in several ways. First, you get closer to the light spectrum that best promotes flower production. second, you get a decent quantity and intensity of added light, which will increase your yield big time. The bulbs are kinda expensive though, so it's up to you. You could also look into getting a red spectrum halide or something. The more red spectrum, the more and especially denser the buds will be.