The biggest potential issue I could see you running into is going to eventually be ventilation. Just one 400W HPS is enough to bring practically any room up to 90+ degrees, so you can probably imagine what two will do. Also, when that room is full of flowering females, smell will most likely become an issue. If you've got the money to spend, you might consider looking into air-cooled hoods or cooltubes for your HPS's, which will work best with an in-line fan. 300-400 CFM would probably keep temperatures down enough, but I'd get the highest CFM you can find that's affordable (and quiet). On top of that, I'd consider attaching a carbon filter to keep the smell manageable.

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