Can a metal halide supplement a hps???

I currently have 8 plants growing under a 400watt HPS. They are all 14 days into flowering. I have a 400watt Metal Halide that I have on stand-by. Would It be ideal for me to use the 400 watt Metal Halide along with the HPS? If so, how would I set it up? Or doesn't not make a difference?

I just wants to know if it's worth adding the MH because of electric bill issues. If the MH is going to help, then I will use it....lol

ANYONE?

I use a 600w hps for flowering and bulk it up with
4*200w red spectrum cfl's at the end of flowering
i have the cfl's in between the plants and i have only
seen positive effects from it,i get a lot more big bud's on the lower branches!!