2 of them do not just 1 core coverage on it is 3 by 3 so 2 400 is at least for a 6 by 6
Uhh... No, 3'x'3 is 9 square feet. 6'x'6 is 36 square feet - which is not twice as large a space, it's four times as large. When you go from one light source to multiple ones, you do (generally) get a "third light" effect, which has a tendency to improve things more than the absolute 2X of just doubling the gross amount of light - but it won't help that much.
A space twice the size of a 3'x3' grow would be 18 square feet. For example, 3'x6' (you may have meant to type that?) or, if a space with a square area, just a tiny fraction of an inch under 4¼' per side.
I hope you do really well with your grow! Not just because I like to see people succeed. I also like to see forum sponsors selling quality products. It's good for the company, it's good for the members, and it's good for the forum. Additionally... There's a G3 HiPAR GR480 heading my way, lol, and so I have a personal reason to hope that GROWant sells great products .