Do you have to buy the extra hps light? Also do you rotate your plants every once in awhile so they are getting a more even range of light? I would say its not needed right now, I use to use a hps and a metal halide in the same hood, I have a dual ballast that runs a 400 watt hps and 400 watt mh at the same time and my plants did very well under the mixed light. I would still use it but I burnt out the mh side of my ballast from running to much on one line awhile back and I never have gotten it fixed. When I switched over to straight hps I had better bud to leaf ratio although Im not sure it helped my yield much but, I do believe I had better trichome production with the mixed light. When I was using the mixed light I was running 1800 watts, 1400 was hps ( a 1000 watt and a 400 watt) and the other 400 being mh. So it's not exactly the same situation, but for what it is worth I do think the plants liked the mixed light better, that being said I don't know if they were more or less productive. I would say keep it as is for now and once you start your next grow try the 2 hps and see which works out better for you.