Buy some CFL's for the Veg room # 18 hrs under CFL is going to be a lot cheaper and you shouldn't need the wattage in veg

You could then double up with the 2x 600wHPS in flower and go for big yields. Maybe using both lights in the last few weeks of flowering so you get the most yield and lowest energy costs.

Messing with the timing will just cause your plants stress, drop yield and likely cause hermy attributes if the plant is stressed and then flowered. I would seriously consider a smaller veg room light.

Just an idea to chop costs! Hell even Envirolights can veg weed nicely. Or invest in a 250w HPS for Veg.