Due to the light spectrum of the HPS you will get taller plants, more stretch, than you will get thick lush tight growth.
You are not looking for maximum height in veg so using a light that will give you that and less better thicker growth is not the best way to go if you have any other option.
Now if your MH will not cover the entire area were plants will be growing then use them both and shift the plants around every few days but if one 400 watt MH light will cover the entire area with adequate lighting just use it.