Personally i think you would be better off just getting a 400 and a 600 W HPS (both). If you just purchase a digital ballast for both lights you can use MH bulbs of the same wattage.
Your thinking on the two lights over one is a hole in one. Im sure there are SOME benefits to useing a (1) 1000W HPS light but thats not why im here. A few reasons why you should use two lights like you said. 400 and 600 watt lights are more efficent then a 1000W light. Also having two points of light , is better then having one bright point of light. If your canopy happens to be uneven you can seperately move the lights up and down without effecting the second light.
In my opinion its kinda useless to use an MH light, Some people will do it to increase the blue spectrum but its just not efficent enough to be used (indoors), compared to two HPS. If you are doing a SOG type of grow (small plants , without much veg time) There is no reason to use anything but HPS lights. If you are growing large bushes (unusual indoors) you may consider the MH light to grow the plants vegetatively. Otherwise you will get the same result by just using HPS lights as your gonna flip to 12/12 to flower ASAP. The HPS lights will give you the buds, and thats what its really all about