The system you have HAS two bulbs. A 400 watt HPS and a 400 watt MH. A lot of people are using such systems these days and instead of the old way where you use the MH totally at first and then the HPS totally to flower, they use both at varying amounts of exposure throughout the grow period. It's said to make bushier more roust growth which means more yields.

But 800 watts burning in that small of an area is going to require some major heat control. Even 400 watts. A cool tube would help big time.