You don't "need" two lights. Yes it is true that plants react differently to certain wavelenghts of light depending on stage of life cycle but you can grow great bud with either hps and mh all the way from seed through veg and flower up until harvest. General opinion is that if you were to choose one light for entire grow you should go with hps because of greater lumen per watt output (although with new HQI mh lumen output is not far behind hps). The end result will be taller/lankier plants as it is the blue part of spectrum telling the plant to stop stretching and hps is missing that. I can explain exactly how it happens in another post if you wish. If you were to grow entirely under mh you can expect slightly smaller harvest (on same wattage when compared with hps) but harder nuggets (plant does not stretch that much so flower growth is naturally more compacted). Another theory is that our plants produce more resin in response to certain wavelenghts of uv light which hps has almost none but metal halide bleeds some even through b/s glass of outer envelope that is there to stop it from getting out. Also - do never go for 5000K light as this one produces least PAR (light usable for plants). Reason being too much of green spectrum in overall light output. We can see it hence lumen rating (describing light visible to us) is the same as other Kelvin values lamps but much of this light is made up of green spectrum which plants can't see, that is why we see green leaves - plants are reflecting most of the light they don't use. 4000K and less or 6000K and more. And with LED well ...... light penetration is overrated and people are getting best results with SCROG because light can be kept closer to ALL budding sites. Nothing to do with technology but with HID, all of your light output comes from a single point (mh) or linear small surface (hps). with LED even with good lenses it is spread out within light fixture. Same wattage shining from a small point produces a lot more penetration. I am not against LEDs but HID is a lot more forgiving and cheaper plus in my opinion produces better results (if you don't believe check latest Subcool test grow with good quality LED compared to hids, even on the plants between two lights buds were bigger/denser on the side lit by hid).
