Its really tough to vent 2 600w without having two air-sealed hoods, and a dedicated intake, coming from outside your house. Having your lamps isolated from the environment makes a huge difference. Thats how I had mine, until I went to led. I had a 6" fan keeping a 1k and 600w cool, and with no effort. My intake came from the crawl space under the house, and vented into the attic. Mind you, this was air that came from outside, then went into the attic, with no added humidity from the plants in the room. Thats why it was okay to exhaust the hood air into to attic, as opposed to exhausting humid air from the grow space.
Do you have a sealed hood?