watch the heat, air cooled hoods connecting both of them would super helpful. But great choice with 2 400's... another note just for info, 600W bulbs are the most energy efficient size per watt you can get in HID, so i personally wouldnt buy bigger than 600 when i could get two smaller bulbs to cover more of a canopy, as well as allow the lights to be closer to the plants
best of luck, and cheers